diff --git a/app/shared/src/commonMain/composeResources/values-ru/strings.xml b/app/shared/src/commonMain/composeResources/values-ru/strings.xml
index f4708aa..5521743 100644
--- a/app/shared/src/commonMain/composeResources/values-ru/strings.xml
+++ b/app/shared/src/commonMain/composeResources/values-ru/strings.xml
@@ -31,6 +31,7 @@
     <string name="display_name_error">От 1 до 64 символов</string>
     <string name="fill_all_fields">Заполните все поля</string>
     <string name="username_length_error">Имя пользователя — от 3 до 20 символов</string>
+    <string name="username_chars_error">Имя пользователя может содержать только английские буквы, цифры, дефисы и подчеркивания</string>
     <string name="password_length_error">Пароль — от 5 до 50 символов</string>
     <string name="passwords_dont_match">Пароли не совпадают</string>
     <string name="auth_welcome_title">Добро пожаловать в FromChat</string>
diff --git a/app/shared/src/commonMain/composeResources/values/strings.xml b/app/shared/src/commonMain/composeResources/values/strings.xml
index 3041ff8..fca2c67 100644
--- a/app/shared/src/commonMain/composeResources/values/strings.xml
+++ b/app/shared/src/commonMain/composeResources/values/strings.xml
@@ -38,6 +38,7 @@
     <!-- Validation Errors -->
     <string name="fill_all_fields">Please fill in every field</string>
     <string name="username_length_error">Username must be 3 to 20 characters</string>
+    <string name="username_chars_error">Username can only contain English letters, numbers, hyphens and underscores</string>
     <string name="password_length_error">Password must be 5 to 50 characters</string>
     <string name="passwords_dont_match">The two passwords don’t match</string>
     <string name="auth_welcome_title">Welcome to FromChat</string>
diff --git a/app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/UsernameStep.kt b/app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/UsernameStep.kt
index d08ad13..140fe65 100644
--- a/app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/UsernameStep.kt
+++ b/app/shared/src/commonMain/kotlin/ru/fromchat/ui/auth/UsernameStep.kt
@@ -42,8 +42,13 @@ import ru.fromchat.ui.components.expressiveStepFieldColors
 import ru.fromchat.ui.components.trackImeScrollTarget
 import ru.fromchat.ui.main.settings.SettingsStepHorizontalPadding
 import ru.fromchat.username
+import ru.fromchat.username_chars_error
 import ru.fromchat.username_length_error
 
+/** Matches backend `is_valid_username`: English letters, digits, hyphen, underscore. */
+private fun isAllowedUsernameChar(ch: Char) =
+    ch in 'a'..'z' || ch in 'A'..'Z' || ch in '0'..'9' || ch == '_' || ch == '-'
+
 @OptIn(ExperimentalMaterial3ExpressiveApi::class)
 @Composable
 internal fun usernameStepPage(
@@ -59,9 +64,11 @@ internal fun usernameStepPage(
 
     val fillAll = stringResource(Res.string.fill_all_fields)
     val usernameLenError = stringResource(Res.string.username_length_error)
+    val usernameCharsError = stringResource(Res.string.username_chars_error)
     val serverFail = stringResource(Res.string.auth_server_connect_failed)
     val unexpected = stringResource(Res.string.error_unexpected)
     val nextLabel = stringResource(Res.string.settings_next)
+    val hasProhibitedChars = username.trim().any { !isAllowedUsernameChar(it) }
 
     return ExpressiveStepPage(
         hero = ExpressiveHeroSpec(
@@ -90,6 +97,12 @@ internal fun usernameStepPage(
                     .trackImeScrollTarget(imeScroll, ExpressiveStepLazyListIndices.STEPS_BODY)
                     .padding(horizontal = SettingsStepHorizontalPadding),
                 singleLine = true,
+                isError = hasProhibitedChars,
+                supportingText = if (hasProhibitedChars) {
+                    { Text(usernameCharsError) }
+                } else {
+                    null
+                },
                 colors = expressiveStepFieldColors(),
                 shape = SettingsPasswordOutlineFieldShape,
             )
@@ -98,7 +111,7 @@ internal fun usernameStepPage(
         button = {
             ActionButton(
                 onClick = {
-                    if (busy) return@ActionButton
+                    if (busy || hasProhibitedChars) return@ActionButton
                     val trimmed = username.trim()
                     if (trimmed.isBlank()) {
                         onSnackbar(fillAll, null)
@@ -136,7 +149,7 @@ internal fun usernameStepPage(
                         }
                     }
                 },
-                enabled = !busy,
+                enabled = !busy && !hasProhibitedChars,
                 loading = busy,
                 modifier = Modifier.fillMaxWidth(),
             ) {