diff --git a/config/config.go b/config/config.go
index d0dacd1a..5714f2d5 100644
--- a/config/config.go
+++ b/config/config.go
@@ -814,7 +814,7 @@ func (r *Route) UnmarshalYAML(unmarshal func(interface{}) error) error {
 			r.GroupByAll = true
 		} else {
 			labelName := model.LabelName(l)
-			if !labelName.IsValid() {
+			if !compat.IsValidLabelName(labelName) {
 				return fmt.Errorf("invalid label name %q in group_by list", l)
 			}
 			r.GroupBy = append(r.GroupBy, labelName)
diff --git a/matchers/compat/parse.go b/matchers/compat/parse.go
index 31cbe626..89b4204d 100644
--- a/matchers/compat/parse.go
+++ b/matchers/compat/parse.go
@@ -16,9 +16,11 @@ package compat
 import (
 	"fmt"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/go-kit/log"
 	"github.com/go-kit/log/level"
+	"github.com/prometheus/common/model"
 
 	"github.com/prometheus/alertmanager/featurecontrol"
 	"github.com/prometheus/alertmanager/matchers/parse"
@@ -26,10 +28,16 @@ import (
 )
 
 var (
-	parseMatcher  = classicMatcherParser(log.NewNopLogger())
-	parseMatchers = classicMatchersParser(log.NewNopLogger())
+	isValidLabelName = isValidClassicLabelName(log.NewNopLogger())
+	parseMatcher     = classicMatcherParser(log.NewNopLogger())
+	parseMatchers    = classicMatchersParser(log.NewNopLogger())
 )
 
+// IsValidLabelName returns true if the string is a valid label name.
+func IsValidLabelName(name model.LabelName) bool {
+	return isValidLabelName(name)
+}
+
 type matcherParser func(s string) (*labels.Matcher, error)
 
 type matchersParser func(s string) (labels.Matchers, error)
@@ -49,12 +57,15 @@ func Matchers(s string) (labels.Matchers, error) {
 // InitFromFlags initializes the compat package from the flagger.
 func InitFromFlags(l log.Logger, f featurecontrol.Flagger) {
 	if f.ClassicMode() {
+		isValidLabelName = isValidClassicLabelName(l)
 		parseMatcher = classicMatcherParser(l)
 		parseMatchers = classicMatchersParser(l)
 	} else if f.UTF8StrictMode() {
+		isValidLabelName = isValidUTF8LabelName(l)
 		parseMatcher = utf8MatcherParser(l)
 		parseMatchers = utf8MatchersParser(l)
 	} else {
+		isValidLabelName = isValidUTF8LabelName(l)
 		parseMatcher = fallbackMatcherParser(l)
 		parseMatchers = fallbackMatchersParser(l)
 	}
@@ -166,3 +177,17 @@ func fallbackMatchersParser(l log.Logger) matchersParser {
 		return m, nil
 	}
 }
+
+// isValidClassicLabelName returns true if the string is a valid classic label name.
+func isValidClassicLabelName(_ log.Logger) func(model.LabelName) bool {
+	return func(name model.LabelName) bool {
+		return name.IsValid()
+	}
+}
+
+// isValidUTF8LabelName returns true if the string is a valid UTF-8 label name.
+func isValidUTF8LabelName(_ log.Logger) func(model.LabelName) bool {
+	return func(name model.LabelName) bool {
+		return utf8.ValidString(string(name))
+	}
+}
diff --git a/matchers/compat/parse_test.go b/matchers/compat/parse_test.go
index d31e275b..ca50c14b 100644
--- a/matchers/compat/parse_test.go
+++ b/matchers/compat/parse_test.go
@@ -17,6 +17,7 @@ import (
 	"testing"
 
 	"github.com/go-kit/log"
+	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/require"
 
 	"github.com/prometheus/alertmanager/pkg/labels"
@@ -110,3 +111,65 @@ func mustNewMatcher(t *testing.T, op labels.MatchType, name, value string) *labe
 	require.NoError(t, err)
 	return m
 }
+
+func TestIsValidClassicLabelName(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    model.LabelName
+		expected bool
+	}{{
+		name:     "is accepted",
+		input:    "foo",
+		expected: true,
+	}, {
+		name:     "is also accepted",
+		input:    "_foo1",
+		expected: true,
+	}, {
+		name:     "is not accepted",
+		input:    "0foo",
+		expected: false,
+	}, {
+		name:     "is also not accepted",
+		input:    "foo🙂",
+		expected: false,
+	}}
+
+	for _, test := range tests {
+		fn := isValidClassicLabelName(log.NewNopLogger())
+		t.Run(test.name, func(t *testing.T) {
+			require.Equal(t, test.expected, fn(test.input))
+		})
+	}
+}
+
+func TestIsValidUTF8LabelName(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    model.LabelName
+		expected bool
+	}{{
+		name:     "is accepted",
+		input:    "foo",
+		expected: true,
+	}, {
+		name:     "is also accepted",
+		input:    "_foo1",
+		expected: true,
+	}, {
+		name:     "is accepted in UTF-8",
+		input:    "0foo",
+		expected: true,
+	}, {
+		name:     "is also accepted with UTF-8",
+		input:    "foo🙂",
+		expected: true,
+	}}
+
+	for _, test := range tests {
+		fn := isValidUTF8LabelName(log.NewNopLogger())
+		t.Run(test.name, func(t *testing.T) {
+			require.Equal(t, test.expected, fn(test.input))
+		})
+	}
+}
diff --git a/test/with_api_v2/acceptance/utf8_test.go b/test/with_api_v2/acceptance/utf8_test.go
index 75d4ecdb..79201847 100644
--- a/test/with_api_v2/acceptance/utf8_test.go
+++ b/test/with_api_v2/acceptance/utf8_test.go
@@ -280,6 +280,8 @@ route:
     - receiver: webhook
       matchers:
         - foo🙂=bar
+      group_by:
+        - foo🙂
       group_wait: 1s
 receivers:
 - name: default