diff --git a/cmd/syncthing/gui.go b/cmd/syncthing/gui.go
index 45721169..5ad34b0a 100644
--- a/cmd/syncthing/gui.go
+++ b/cmd/syncthing/gui.go
@@ -1328,7 +1328,7 @@ func addressIsLocalhost(addr string) bool {
 		// There was no port, so we assume the address was just a hostname
 		host = addr
 	}
-	switch host {
+	switch strings.ToLower(host) {
 	case "127.0.0.1", "::1", "localhost":
 		return true
 	default:
diff --git a/cmd/syncthing/gui_test.go b/cmd/syncthing/gui_test.go
index 961e66ca..0b5d4533 100644
--- a/cmd/syncthing/gui_test.go
+++ b/cmd/syncthing/gui_test.go
@@ -833,9 +833,11 @@ func TestAddressIsLocalhost(t *testing.T) {
 	}{
 		// These are all valid localhost addresses
 		{"localhost", true},
+		{"LOCALHOST", true},
 		{"::1", true},
 		{"127.0.0.1", true},
 		{"localhost:8080", true},
+		{"LOCALHOST:8000", true},
 		{"[::1]:8080", true},
 		{"127.0.0.1:8080", true},