diff --git a/.deadcode-out b/.deadcode-out
index 97093ce93b..6d2c35e374 100644
--- a/.deadcode-out
+++ b/.deadcode-out
@@ -23,6 +23,7 @@ forgejo.org/models/db
 	DumpTables
 	GetTableNames
 	extendBeansForCascade
+	IsErrNameActivityPubInvalid
 
 forgejo.org/models/dbfs
 	file.renameTo
diff --git a/models/db/name.go b/models/db/name.go
index 29b60b2373..d456f49d9c 100644
--- a/models/db/name.go
+++ b/models/db/name.go
@@ -80,6 +80,31 @@ func (err ErrNameCharsNotAllowed) Unwrap() error {
 	return util.ErrInvalidArgument
 }
 
+// ErrNameActivityPubInvalid represents an error for usernames which cannot
+// belong to ActivityPub accounts.
+type ErrNameActivityPubInvalid struct {
+	Name string
+}
+
+// Similarly to IsErrNameCharsNotAllowed, IsErrNameActivityPubInvalid checks if
+// an error is an ErrNameActivityPubInvalid.
+func IsErrNameActivityPubInvalid(err error) bool {
+	_, ok := err.(ErrNameActivityPubInvalid)
+	return ok
+}
+
+func (err ErrNameActivityPubInvalid) Error() string {
+	return fmt.Sprintf(
+		"name is invalid [%s]: not acceptable for users from federated activitypub instances (e.g. @username@domain.example)",
+		err.Name,
+	)
+}
+
+// Unwrap unwraps this as a ErrInvalidArgument err
+func (err ErrNameActivityPubInvalid) Unwrap() error {
+	return util.ErrInvalidArgument
+}
+
 // IsUsableName checks if name is reserved or pattern of name is not allowed
 // based on given reserved names and patterns.
 // Names are exact match, patterns can be prefix or suffix match with placeholder '*'.
diff --git a/models/fixtures/user.yml b/models/fixtures/user.yml
index 4e957f30f3..17592ef330 100644
--- a/models/fixtures/user.yml
+++ b/models/fixtures/user.yml
@@ -1566,9 +1566,9 @@
 
 -
   id: 42
-  lower_name: federated-example.net
-  name: federated-example.net
-  full_name: federated
+  lower_name: "@federated@example.net"
+  name: "@federated@example.net"
+  full_name: "@federated@example.net"
   email: f73240e82-c061-41ef-b7d6-4376cb6f2e1c@example.com
   keep_email_private: false
   email_notifications_preference: enabled
@@ -1576,7 +1576,7 @@
   passwd_hash_algo: ""
   must_change_password: false
   login_source: 0
-  login_name: federated-example.net
+  login_name: "@federated@example.net"
   type: 6
   salt: ""
   max_repo_creation: -1
diff --git a/models/forgejo_migrations/v14a_ap-change-fedi-handle-structure.go b/models/forgejo_migrations/v14a_ap-change-fedi-handle-structure.go
new file mode 100644
index 0000000000..c59f778205
--- /dev/null
+++ b/models/forgejo_migrations/v14a_ap-change-fedi-handle-structure.go
@@ -0,0 +1,150 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package forgejo_migrations
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"forgejo.org/models/db"
+	"forgejo.org/models/forgefed"
+	user_model "forgejo.org/models/user"
+	"forgejo.org/modules/log"
+	"forgejo.org/modules/validation"
+	user_service "forgejo.org/services/user"
+
+	"xorm.io/xorm"
+)
+
+func init() {
+	registerMigration(&Migration{
+		Description: "use structure @PreferredUsername@host.tld:port for actors",
+		Upgrade:     changeActivityPubUsernameFormat,
+	})
+}
+
+func changeActivityPubUsernameFormat(x *xorm.Engine) error {
+	// Normally, the db.WithTx statement ensures that the database transaction (aka. all changes made
+	// by this migration) will only be committed if the SQL operations inside of the iteration
+	// (db.Iterate) don't return an error.
+	//
+	// This migration was originally authored with those cases in mind, but it was later agreed that
+	// migrations concerning Forgejo's federation-related components should not return any errors at
+	// this point in time, as federation is not considered to be stable at the moment. For more
+	// information, check the relevant discussion here:
+	// https://codeberg.org/forgejo-contrib/federation/issues/67
+	//
+	// Nevertheless, this structure involves some useful boilerplate that can be used for future
+	// migrations at a later point and has been kept as-is.
+	return db.WithTx(db.DefaultContext, func(ctx context.Context) error {
+		// The transaction is committed only if modifying all federated users is possible.
+		return db.Iterate(ctx, nil, func(ctx context.Context, federatedUser *user_model.FederatedUser) error {
+			// localUser represents the "local" representation of an ActivityPub (federated) user
+			localUser := &user_model.User{}
+			has, err := db.GetEngine(ctx).ID(federatedUser.UserID).Get(localUser)
+			if err != nil {
+				log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred while getting local user (ID: %d), ignoring...: %e", federatedUser.UserID, err)
+				return nil
+			}
+
+			if !has {
+				log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: User missing for federated user: %v", federatedUser)
+				err := user_model.DeleteFederatedUser(ctx, federatedUser.UserID)
+				if err != nil {
+					log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred while deleting federated user (%s), ignoring...: %e", federatedUser, err)
+					return nil
+				}
+			}
+
+			if validation.IsValidActivityPubUsername(localUser.Name) {
+				log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: FederatedUser was already migrated: %v", federatedUser)
+			} else {
+				// Copied from models/forgefed/federationhost_repository.go (forgefed.GetFederationHost),
+				// minus some validation code for FederationHost which we do not otherwise manipulate here.
+				federationHost := new(forgefed.FederationHost)
+				has, err := db.GetEngine(ctx).ID(federatedUser.FederationHostID).Get(federationHost)
+				if err != nil {
+					log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred while looking up federation host info (for %v), ignoring...: %e", federatedUser, err)
+					return nil
+				} else if !has {
+					log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Federation host for federated user missing, deleting: %v", federatedUser)
+					err := user_model.DeleteFederatedUser(ctx, federatedUser.UserID)
+					if err != nil {
+						log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred while deleting federated user (%v), ignoring...: %e", federatedUser, err)
+						return nil
+					}
+
+					err = user_service.DeleteUser(ctx, localUser, true)
+					if err != nil {
+						log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred while deleting user (%s), ignoring...: %v", localUser.LogString(), err)
+					}
+
+					return nil
+				}
+
+				// Take part of the username before the first dash, reconstruct the rest
+				// of it using whatever we have in FederationHost. Before this migration,
+				// usernames of ActivityPub accounts have an expected format of
+				// "username-subdomain-domain-tld-port". We don't know how many subdomains
+				// there are, but that doesn't matter. We can always get the username unless
+				// if the username of an ActivityPub account was manually changed by an admin,
+				// in which case they should either delete the account or change it back.
+				s := strings.Split(localUser.Name, "-")
+				if len(s) == 0 {
+					log.Warn(
+						"Migration[v14a_ap-change-fedi-handle-structure]: Username %s belonging to federatedUser %v does not contain any dashes, can't construct new username",
+						localUser.Name,
+						federatedUser,
+					)
+					return nil
+				}
+
+				// Were a running Forgejo instance to create a new federated account, would the port
+				// have been marked as "supplemented" (thus leading to its omission)?
+				var newUsername string
+				if (federationHost.HostPort == 443 && federationHost.HostSchema == "https") || (federationHost.HostPort == 80 && federationHost.HostSchema == "http") {
+					newUsername = fmt.Sprintf("@%s@%s", s[0], federationHost.HostFqdn)
+				} else {
+					newUsername = fmt.Sprintf("@%s@%s:%d", s[0], federationHost.HostFqdn, federationHost.HostPort)
+				}
+
+				// Implicitly assumes that there won't be a lower name unique constraint violation.
+				// Potentially a bit paranoid, but why not?
+				userThatShouldntExist := &user_model.User{}
+				lowernameTaken, err := db.GetEngine(ctx).Where("lower_name = ?", strings.ToLower(newUsername)).Table("user").Get(userThatShouldntExist)
+				if err != nil {
+					log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred, skipping migration of %s: %e", localUser.LogString(), err)
+					return nil
+				}
+
+				if lowernameTaken {
+					log.Warn(
+						"Migration[v14a_ap-change-fedi-handle-structure]: New username %s for %s already taken by %s, deleting the former...",
+						newUsername,
+						localUser.LogString(),
+						userThatShouldntExist.LogString(),
+					)
+					err := user_model.DeleteFederatedUser(ctx, localUser.ID)
+					if err != nil {
+						log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred while deleting federated user (%s), ignoring...: %e", localUser.LogString(), err)
+					}
+					return nil
+				}
+
+				// Safe to assume that the following operations should just work now.
+				log.Info("Migration[v14a_ap-change-fedi-handle-structure]: Updating username of %s to %s", localUser.LogString(), newUsername)
+				if _, err := db.GetEngine(ctx).ID(localUser.ID).Cols("lower_name", "name").Update(&user_model.User{
+					LowerName: strings.ToLower(newUsername),
+					Name:      newUsername,
+				}); err != nil {
+					log.Warn("Migration[v14a_ap-change-fedi-handle-structure]: Database error occurred when updating federated user's username (%s), ignoring...: %e", localUser.LogString(), err)
+					return nil
+				}
+			}
+
+			return nil
+		})
+	})
+}
diff --git a/models/user/user.go b/models/user/user.go
index 0fd8061a89..cc5b4abec3 100644
--- a/models/user/user.go
+++ b/models/user/user.go
@@ -698,6 +698,14 @@ func IsUsableUsername(name string) error {
 	return db.IsUsableName(reservedUsernames, reservedUserPatterns, name)
 }
 
+// IsActivityPubUsername returns an error if a fediverse handle (referred to as a username) cannot exist
+func IsActivityPubUsername(name string) error {
+	if !validation.IsValidActivityPubUsername(name) {
+		return db.ErrNameActivityPubInvalid{Name: name}
+	}
+	return db.IsUsableName(reservedUsernames, reservedUserPatterns, name)
+}
+
 // CreateUserOverwriteOptions are an optional options who overwrite system defaults on user creation
 type CreateUserOverwriteOptions struct {
 	KeepEmailPrivate             optional.Option[bool]
@@ -708,6 +716,7 @@ type CreateUserOverwriteOptions struct {
 	Theme                        *string
 	IsRestricted                 optional.Option[bool]
 	IsActive                     optional.Option[bool]
+	IsActivityPub                optional.Option[bool]
 }
 
 // CreateUser creates record of a new user.
@@ -722,12 +731,26 @@ func AdminCreateUser(ctx context.Context, u *User, overwriteDefault ...*CreateUs
 
 // createUser creates record of a new user.
 func createUser(ctx context.Context, u *User, createdByAdmin bool, overwriteDefault ...*CreateUserOverwriteOptions) (err error) {
-	if err = IsUsableUsername(u.Name); err != nil {
+	overwriteDefaultPresent := len(overwriteDefault) != 0 && overwriteDefault[0] != nil
+
+	// If a username is invalid as-is, check whether the username is meant
+	// for an ActivityPub account. Username constraints that belong to "foreign"
+	// ActivityPub servers, whose implementations we cannot control, are expected
+	// to be much less restrictive than those of Forgejo itself.
+	if overwriteDefaultPresent && overwriteDefault[0].IsActivityPub.Has() {
+		if err = IsActivityPubUsername(u.Name); err != nil {
+			return err
+		}
+	} else if err := IsUsableUsername(u.Name); err != nil {
 		return err
 	}
 
 	// Check if the new username can be claimed.
 	// Skip this check if done by an admin.
+	//
+	// Note: This skip should not currently cover usernames that could belong to
+	// fediverse accounts. This "defensive programming" is in place to prevent future
+	// breakage until the ActivityPub component matures more.
 	if !createdByAdmin {
 		if ok, expireTime, err := CanClaimUsername(ctx, u.Name, -1); err != nil {
 			return err
@@ -754,7 +777,7 @@ func createUser(ctx context.Context, u *User, createdByAdmin bool, overwriteDefa
 	}
 
 	// overwrite defaults if set
-	if len(overwriteDefault) != 0 && overwriteDefault[0] != nil {
+	if overwriteDefaultPresent {
 		overwrite := overwriteDefault[0]
 		if overwrite.KeepEmailPrivate.Has() {
 			u.KeepEmailPrivate = overwrite.KeepEmailPrivate.Value()
diff --git a/models/user/user_repository.go b/models/user/user_repository.go
index f1d06abe17..9692bd7304 100644
--- a/models/user/user_repository.go
+++ b/models/user/user_repository.go
@@ -23,8 +23,9 @@ func CreateFederatedUser(ctx context.Context, user *User, federatedUser *Federat
 		return err
 	}
 	overwrite := CreateUserOverwriteOptions{
-		IsActive:     optional.Some(false),
-		IsRestricted: optional.Some(false),
+		IsActive:      optional.Some(false),
+		IsRestricted:  optional.Some(false),
+		IsActivityPub: optional.Some(true),
 	}
 
 	// Begin transaction
diff --git a/models/user/user_test.go b/models/user/user_test.go
index 330a3fd563..4db253df18 100644
--- a/models/user/user_test.go
+++ b/models/user/user_test.go
@@ -440,6 +440,63 @@ func TestCreateUserClaimingUsername(t *testing.T) {
 	})
 }
 
+// Attempts to create a username with a fediverse-format handle, which should
+// fail (without the override IsActivityPub, which is set by CreateFederatedUser)
+func TestCreateUserPlainWithFediverseHandle(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+
+	_, err := db.GetEngine(db.DefaultContext).NoAutoTime().Insert(&user_model.Redirect{RedirectUserID: 1, LowerName: "redirecting", CreatedUnix: timeutil.TimeStampNow()})
+	require.NoError(t, err)
+
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+
+	user.Name = "@example@example.tld"
+	user.LowerName = strings.ToLower(user.Name)
+	user.ID = 0
+	user.Email = "unique@example.com"
+
+	t.Run("Normal creation (without ActivityPub override)", func(t *testing.T) {
+		err = user_model.CreateUser(db.DefaultContext, user)
+		require.Error(t, err)
+		assert.True(t, db.IsErrNameCharsNotAllowed(err))
+	})
+
+	t.Run("Creation as admin (without ActivityPub override)", func(t *testing.T) {
+		err = user_model.AdminCreateUser(db.DefaultContext, user)
+		require.Error(t, err)
+		assert.True(t, db.IsErrNameCharsNotAllowed(err))
+	})
+
+	// Logic borrowed from CreateFederatedUser (which invokes CreateUser), but
+	// we "lend" this here to verify CreateUser's paths.
+	overwrite := user_model.CreateUserOverwriteOptions{
+		IsActive:      optional.Some(false),
+		IsRestricted:  optional.Some(false),
+		IsActivityPub: optional.Some(true),
+	}
+
+	t.Run("Normal creation (with ActivityPub override, invalid format)", func(t *testing.T) {
+		user.Name = "invalid-format-for-an-activitypub-account"
+		user.LowerName = strings.ToLower(user.Name)
+
+		err = user_model.CreateUser(db.DefaultContext, user, &overwrite)
+		require.Error(t, err)
+		assert.True(t, db.IsErrNameActivityPubInvalid(err))
+	})
+
+	t.Run("Normal creation (with ActivityPub override)", func(t *testing.T) {
+		user.Name = "@valid@example.tld"
+		user.LowerName = strings.ToLower(user.Name)
+
+		err = user_model.CreateUser(db.DefaultContext, user, &overwrite)
+		require.NoError(t, err)
+	})
+
+	// Note: We don't expect that admins are able to access any front-facing
+	// function that sets the overwrite (i.e. CreateFederatedUser), hence it
+	// has been omitted for now.
+}
+
 func TestGetUserIDsByNames(t *testing.T) {
 	require.NoError(t, unittest.PrepareTestDatabase())
 
diff --git a/modules/forgefed/actor_person.go b/modules/forgefed/actor_person.go
index 7c43b0d7ce..72768f4815 100644
--- a/modules/forgefed/actor_person.go
+++ b/modules/forgefed/actor_person.go
@@ -71,12 +71,13 @@ func (id PersonID) AsLoginName() string {
 	return result
 }
 
+// HostSuffix returns the host part of a handle, i.e. @host.tld (if port is supplemented) or @host.tld:1234
 func (id PersonID) HostSuffix() string {
 	var result string
 	if !id.IsPortSupplemented {
-		result = fmt.Sprintf("-%s-%d", strings.ToLower(id.Host), id.HostPort)
+		result = fmt.Sprintf("@%s:%d", strings.ToLower(id.Host), id.HostPort)
 	} else {
-		result = fmt.Sprintf("-%s", strings.ToLower(id.Host))
+		result = fmt.Sprintf("@%s", strings.ToLower(id.Host))
 	}
 	return result
 }
diff --git a/modules/forgefed/actor_person_test.go b/modules/forgefed/actor_person_test.go
index a5f3ee47b1..82bd9fabc3 100644
--- a/modules/forgefed/actor_person_test.go
+++ b/modules/forgefed/actor_person_test.go
@@ -246,8 +246,19 @@ func TestForgePersonValidation(t *testing.T) {
 
 func TestAsloginName(t *testing.T) {
 	sut, _ := forgefed.NewPersonID("https://codeberg.org/api/v1/activitypub/user-id/12345", "forgejo")
-	assert.Equal(t, "12345-codeberg.org", sut.AsLoginName())
+	assert.Equal(t, "12345@codeberg.org", sut.AsLoginName())
 
 	sut, _ = forgefed.NewPersonID("https://codeberg.org:443/api/v1/activitypub/user-id/12345", "forgejo")
-	assert.Equal(t, "12345-codeberg.org-443", sut.AsLoginName())
+	assert.Equal(t, "12345@codeberg.org:443", sut.AsLoginName())
+}
+
+func TestHostSuffix(t *testing.T) {
+	sut, _ := forgefed.NewPersonID("https://codeberg.org/api/v1/activitypub/user-id/12345", "forgejo")
+	sut.Host = "forgejo.example.tld"
+	sut.HostPort = 80
+
+	// sut.IsPortSupplemented is true by default at time of writing.
+	assert.Equal(t, "@forgejo.example.tld", sut.HostSuffix())
+	sut.IsPortSupplemented = false
+	assert.Equal(t, "@forgejo.example.tld:80", sut.HostSuffix())
 }
diff --git a/modules/validation/helpers.go b/modules/validation/helpers.go
index 4b28dead03..848fb70af5 100644
--- a/modules/validation/helpers.go
+++ b/modules/validation/helpers.go
@@ -102,6 +102,21 @@ var (
 
 	// No consecutive or trailing non-alphanumeric chars, catches both cases
 	invalidUsernamePattern = regexp.MustCompile(`[-._]{2,}|[-._]$`)
+
+	// This is intended to accept any character, in any language, with accent symbols,
+	// as well as an arbitrary amount of subdomains and an optional port number defined
+	// through `:12345`.
+	//
+	// This is intended to cover username cases from distant servers in the fediverse, which
+	// can have much laxer requirements than those of Forgejo. It is not intended to check for
+	// invalid, non-standard compliant domains.
+	//
+	// For instance, the following should work:
+	// @user.όνομαß_21__@subdomain1.subdomain2.example.tld:65536
+	// @42@42.example.tld
+	// @user@example.tld:99999 (presumed to be an impossible case)
+	// @-@-.tld (also impossible)
+	validFediverseUsernamePattern = regexp.MustCompile(`^(@[\p{L}\p{M}0-9_\.\-]{1,})(@[\p{L}\p{M}0-9_\.\-]{1,})(:[1-9][0-9]{0,4})?$`)
 )
 
 // IsValidUsername checks if username is valid
@@ -114,3 +129,11 @@ func IsValidUsername(name string) bool {
 
 	return validUsernamePatternWithoutDots.MatchString(name) && !invalidUsernamePattern.MatchString(name)
 }
+
+// IsValidActivityPubUsername checks whether the username can be a valid ActivityPub handle.
+//
+// Username refers to the Forgejo user account's username for consistency, and not
+// e.g. "username" in @username@example.tld.
+func IsValidActivityPubUsername(name string) bool {
+	return validFediverseUsernamePattern.MatchString(name)
+}
diff --git a/modules/validation/helpers_test.go b/modules/validation/helpers_test.go
index 7e32184691..6bca3664e1 100644
--- a/modules/validation/helpers_test.go
+++ b/modules/validation/helpers_test.go
@@ -1,4 +1,5 @@
 // Copyright 2018 The Gitea Authors. All rights reserved.
+// Copyright 2025 The Forgejo Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
 package validation
@@ -213,3 +214,109 @@ func TestIsValidUsernameBanDots(t *testing.T) {
 		})
 	}
 }
+
+func TestIsValidActivityPubUsername(t *testing.T) {
+	cases := []struct {
+		description string
+		username    string
+		valid       bool
+	}{
+		{
+			description: "Username without domain",
+			username:    "@user",
+			valid:       false,
+		},
+		{
+			description: "Username with domain",
+			username:    "@user@example.tld",
+			valid:       true,
+		},
+		{
+			description: "Numeric username with subdomain",
+			username:    "@42@42.example.tld",
+			valid:       true,
+		},
+		{
+			description: "Username with two subdomains",
+			username:    "@user@forgejo.activitypub.example.tld",
+			valid:       true,
+		},
+		{
+			description: "Username with domain and without port",
+			username:    "@user@social.example.tld:",
+			valid:       false,
+		},
+		{
+			description: "Username with domain and invalid port 0",
+			username:    "@user@social.example.tld:0",
+			valid:       false,
+		},
+		{
+			// We do not validate the port and assume that federationHost.HostPort
+			// cannot present such invalid ports. That also makes the previous case
+			// (port: 0) redundant, but it doesn't hurt.
+			description: "Username with domain and valid port",
+			username:    "@user@social.example.tld:65536",
+			valid:       true,
+		},
+		{
+			description: "Username with Latin letters and special symbols",
+			username:    "@$username$@example.tld",
+			valid:       false,
+		},
+		{
+			description: "Strictly numeric handle, domain, TLD",
+			username:    "@0123456789@0123456789.0123456789.123",
+			valid:       true,
+		},
+		{
+			description: "Handle with Latin characters and dashes",
+			username:    "@0-O@O-O.tld",
+			valid:       true,
+		},
+		// This is an impossible case, but we assume that this will never happen
+		// to begin with.
+		{
+			description: "Handle that only has dashes",
+			username:    "@-@-.-",
+			valid:       true,
+		},
+		{
+			description: "Username with a mix of Latin and non-Latin letters containing accents",
+			username:    "@usernäme.όνομαß_21__@example.tld",
+			valid:       true,
+		},
+		// Note: Our regex should accept any character, in any language and with accent symbols.
+		// The list is neither exhaustive, nor does it represent all possible cases.
+		// I chose some TLDs from https://en.wikipedia.org/wiki/Country_code_top-level_domain,
+		// although only one test case should suffice in theory. Nevertheless, to play it safe,
+		// I included four from different geographic regions whose scripts were legible using my
+		// IDE's default font to play it safe.
+		{
+			description: "Username, domain and ccTLD in Greek",
+			username:    "@ευ@ευ.ευ",
+			valid:       true,
+		},
+		{
+			description: "Username, domain and ccTLD in Georgian (Mkhedruli)",
+			username:    "@გე@გე.გე",
+			valid:       true,
+		},
+		{
+			description: "Username, domain and ccTLD of Malaysia (Arabic Jawi)",
+			username:    "@مليسيا@ລمليسيا.مليسيا",
+			valid:       true,
+		},
+		{
+			description: "Username, domain and ccTLD of China (Simplified)",
+			username:    "@中国@中国.中国",
+			valid:       true,
+		},
+	}
+
+	for _, testCase := range cases {
+		t.Run(testCase.description, func(t *testing.T) {
+			assert.Equal(t, testCase.valid, IsValidActivityPubUsername(testCase.username))
+		})
+	}
+}
diff --git a/services/federation/federation_service.go b/services/federation/federation_service.go
index 69b2379cce..d0336881a8 100644
--- a/services/federation/federation_service.go
+++ b/services/federation/federation_service.go
@@ -173,7 +173,7 @@ func fetchUserFromAP(ctx context.Context, personID fm.PersonID, federationHostID
 
 	email := fmt.Sprintf("f%v@%v", uuid.New().String(), localFqdn.Hostname())
 	loginName := personID.AsLoginName()
-	name := fmt.Sprintf("%v%v", person.PreferredUsername.String(), personID.HostSuffix())
+	name := fmt.Sprintf("@%v%v", person.PreferredUsername.String(), personID.HostSuffix())
 	fullName := person.Name.String()
 
 	if len(person.Name) == 0 {
diff --git a/tests/integration/admin_user_test.go b/tests/integration/admin_user_test.go
index 02d5898826..611a0e89c6 100644
--- a/tests/integration/admin_user_test.go
+++ b/tests/integration/admin_user_test.go
@@ -171,11 +171,11 @@ func TestSourceId(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 
 	testUser23 := &user_model.User{
-		Name:        "ausersourceid23",
-		LoginName:   "ausersourceid23",
+		Name:        "@ausersourceid23@example.net",
+		LoginName:   "@ausersourceid23@example.net",
 		Email:       "ausersourceid23@example.com",
 		Passwd:      "ausersourceid23password",
-		Type:        user_model.UserTypeIndividual,
+		Type:        user_model.UserTypeRemoteUser,
 		LoginType:   auth_model.Plain,
 		LoginSource: 23,
 	}
@@ -184,13 +184,17 @@ func TestSourceId(t *testing.T) {
 	session := loginUser(t, "user1")
 	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeReadAdmin)
 
-	// Our new user start with 'a' so it should be the first one
+	// Historic background: The user was previously called "ausersourceid23", but the
+	// test started failing on PostgreSQL specifically because of another federated user
+	// in a fixture called @federated@example.net - this did not apply to other database
+	// engines. Said user's username began with an 'a' so that it comes up on top, so, we
+	// simply made another federated user that starts with '@a' here as an easy way out.
 	req := NewRequest(t, "GET", "/api/v1/admin/users?limit=1").AddTokenAuth(token)
 	resp := session.MakeRequest(t, req, http.StatusOK)
 	var users []api.User
 	DecodeJSON(t, resp, &users)
 	assert.Len(t, users, 1)
-	assert.Equal(t, "ausersourceid23", users[0].UserName)
+	assert.Equal(t, "@ausersourceid23@example.net", users[0].UserName)
 
 	// Now our new user should not be in the list, because we filter by source_id 0
 	req = NewRequest(t, "GET", "/api/v1/admin/users?limit=1&source_id=0").AddTokenAuth(token)
@@ -204,7 +208,7 @@ func TestSourceId(t *testing.T) {
 	resp = session.MakeRequest(t, req, http.StatusOK)
 	DecodeJSON(t, resp, &users)
 	assert.Len(t, users, 1)
-	assert.Equal(t, "ausersourceid23", users[0].UserName)
+	assert.Equal(t, "@ausersourceid23@example.net", users[0].UserName)
 }
 
 func TestAdminViewUsersSorted(t *testing.T) {