diff --git a/promql/parser/printer.go b/promql/parser/printer.go
index 961167428b..67b13eaf12 100644
--- a/promql/parser/printer.go
+++ b/promql/parser/printer.go
@@ -109,7 +109,7 @@ func writeLabels(b *bytes.Buffer, ss []string) {
 		if i > 0 {
 			b.WriteString(", ")
 		}
-		if !model.LegacyValidation.IsValidMetricName(s) {
+		if !model.LegacyValidation.IsValidLabelName(s) {
 			b.Write(strconv.AppendQuote(b.AvailableBuffer(), s))
 		} else {
 			b.WriteString(s)
@@ -145,6 +145,19 @@ func (node *BinaryExpr) ShortString() string {
 	return node.Op.String() + node.returnBool() + node.getMatchingStr()
 }
 
+// joinLabels joins label names, quoting them if they are not valid legacy label names.
+func joinLabels(labels []string) string {
+	quoted := make([]string, 0, len(labels))
+	for _, label := range labels {
+		if model.LegacyValidation.IsValidLabelName(label) {
+			quoted = append(quoted, label)
+		} else {
+			quoted = append(quoted, strconv.Quote(label))
+		}
+	}
+	return strings.Join(quoted, ", ")
+}
+
 func (node *BinaryExpr) getMatchingStr() string {
 	matching := ""
 	vm := node.VectorMatching
@@ -154,7 +167,7 @@ func (node *BinaryExpr) getMatchingStr() string {
 			if vm.On {
 				vmTag = "on"
 			}
-			matching = fmt.Sprintf(" %s (%s)", vmTag, strings.Join(vm.MatchingLabels, ", "))
+			matching = fmt.Sprintf(" %s (%s)", vmTag, joinLabels(vm.MatchingLabels))
 		}
 
 		if vm.Card == CardManyToOne || vm.Card == CardOneToMany {
@@ -162,7 +175,7 @@ func (node *BinaryExpr) getMatchingStr() string {
 			if vm.Card == CardManyToOne {
 				vmCard = "left"
 			}
-			matching += fmt.Sprintf(" group_%s (%s)", vmCard, strings.Join(vm.Include, ", "))
+			matching += fmt.Sprintf(" group_%s (%s)", vmCard, joinLabels(vm.Include))
 		}
 	}
 	return matching
diff --git a/promql/parser/printer_test.go b/promql/parser/printer_test.go
index b28da988da..c9a3cb35e8 100644
--- a/promql/parser/printer_test.go
+++ b/promql/parser/printer_test.go
@@ -269,6 +269,10 @@ func TestExprString(t *testing.T) {
 		{
 			in: `predict_linear(foo[1h], 3000)`,
 		},
+		{
+			in:  `sum by("üüü") (foo)`,
+			out: `sum by ("üüü") (foo)`,
+		},
 	}
 
 	EnableExtendedRangeSelectors = true
@@ -394,3 +398,45 @@ func TestVectorSelector_String(t *testing.T) {
 		})
 	}
 }
+
+func TestBinaryExprUTF8Labels(t *testing.T) {
+	testCases := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "UTF-8 labels in on clause",
+			input:    `foo / on("äää") bar`,
+			expected: `foo / on ("äää") bar`,
+		},
+		{
+			name:     "UTF-8 labels in group_left clause",
+			input:    `foo / on("äää") group_left("ööö") bar`,
+			expected: `foo / on ("äää") group_left ("ööö") bar`,
+		},
+		{
+			name:     "Mixed legacy and UTF-8 labels",
+			input:    `foo / on(legacy, "üüü") bar`,
+			expected: `foo / on (legacy, "üüü") bar`,
+		},
+		{
+			name:     "Legacy labels only (should not quote)",
+			input:    `foo / on(job, instance) bar`,
+			expected: `foo / on (job, instance) bar`,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			expr, err := ParseExpr(tc.input)
+			if err != nil {
+				t.Fatalf("Failed to parse: %v", err)
+			}
+			result := expr.String()
+			if result != tc.expected {
+				t.Errorf("Expected: %s\nGot: %s", tc.expected, result)
+			}
+		})
+	}
+}