diff --git a/src/Identity/Extensions.Core/src/IdentityErrorDescriber.cs b/src/Identity/Extensions.Core/src/IdentityErrorDescriber.cs
index 919ea59f4288..7e8787551146 100644
--- a/src/Identity/Extensions.Core/src/IdentityErrorDescriber.cs
+++ b/src/Identity/Extensions.Core/src/IdentityErrorDescriber.cs
@@ -95,13 +95,14 @@ public virtual IdentityError LoginAlreadyAssociated()
     /// Returns an <see cref="IdentityError"/> indicating the specified user <paramref name="userName"/> is invalid.
     /// </summary>
     /// <param name="userName">The user name that is invalid.</param>
+    /// <param name="allowedUsernameCharacters">The characters that are allowed in the username.</param>
     /// <returns>An <see cref="IdentityError"/> indicating the specified user <paramref name="userName"/> is invalid.</returns>
-    public virtual IdentityError InvalidUserName(string? userName)
+    public virtual IdentityError InvalidUserName(string? userName, string? allowedUsernameCharacters)
     {
         return new IdentityError
         {
             Code = nameof(InvalidUserName),
-            Description = Resources.FormatInvalidUserName(userName)
+            Description = Resources.FormatInvalidUserName(userName, allowedUsernameCharacters)
         };
     }
 
diff --git a/src/Identity/Extensions.Core/src/PublicAPI.Shipped.txt b/src/Identity/Extensions.Core/src/PublicAPI.Shipped.txt
index b30ce145ace6..6b99eef2de8c 100644
--- a/src/Identity/Extensions.Core/src/PublicAPI.Shipped.txt
+++ b/src/Identity/Extensions.Core/src/PublicAPI.Shipped.txt
@@ -387,7 +387,7 @@ virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.DuplicateUserName(s
 virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.InvalidEmail(string? email) -> Microsoft.AspNetCore.Identity.IdentityError!
 virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.InvalidRoleName(string? role) -> Microsoft.AspNetCore.Identity.IdentityError!
 virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.InvalidToken() -> Microsoft.AspNetCore.Identity.IdentityError!
-virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.InvalidUserName(string? userName) -> Microsoft.AspNetCore.Identity.IdentityError!
+virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.InvalidUserName(string? userName, string? allowedUsernameCharacters) -> Microsoft.AspNetCore.Identity.IdentityError!
 virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.LoginAlreadyAssociated() -> Microsoft.AspNetCore.Identity.IdentityError!
 virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.PasswordMismatch() -> Microsoft.AspNetCore.Identity.IdentityError!
 virtual Microsoft.AspNetCore.Identity.IdentityErrorDescriber.PasswordRequiresDigit() -> Microsoft.AspNetCore.Identity.IdentityError!
diff --git a/src/Identity/Extensions.Core/src/Resources.resx b/src/Identity/Extensions.Core/src/Resources.resx
index fe35a8b7868a..46406e5db842 100644
--- a/src/Identity/Extensions.Core/src/Resources.resx
+++ b/src/Identity/Extensions.Core/src/Resources.resx
@@ -1,17 +1,17 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <root>
-  <!-- 
-    Microsoft ResX Schema 
-    
+  <!--
+    Microsoft ResX Schema
+
     Version 2.0
-    
-    The primary goals of this format is to allow a simple XML format 
-    that is mostly human readable. The generation and parsing of the 
-    various data types are done through the TypeConverter classes 
+
+    The primary goals of this format is to allow a simple XML format
+    that is mostly human readable. The generation and parsing of the
+    various data types are done through the TypeConverter classes
     associated with the data types.
-    
+
     Example:
-    
+
     ... ado.net/XML headers & schema ...
     <resheader name="resmimetype">text/microsoft-resx</resheader>
     <resheader name="version">2.0</resheader>
@@ -26,36 +26,36 @@
         <value>[base64 mime encoded string representing a byte array form of the .NET Framework object]</value>
         <comment>This is a comment</comment>
     </data>
-                
-    There are any number of "resheader" rows that contain simple 
+
+    There are any number of "resheader" rows that contain simple
     name/value pairs.
-    
-    Each data row contains a name, and value. The row also contains a 
-    type or mimetype. Type corresponds to a .NET class that support 
-    text/value conversion through the TypeConverter architecture. 
-    Classes that don't support this are serialized and stored with the 
+
+    Each data row contains a name, and value. The row also contains a
+    type or mimetype. Type corresponds to a .NET class that support
+    text/value conversion through the TypeConverter architecture.
+    Classes that don't support this are serialized and stored with the
     mimetype set.
-    
-    The mimetype is used for serialized objects, and tells the 
-    ResXResourceReader how to depersist the object. This is currently not 
+
+    The mimetype is used for serialized objects, and tells the
+    ResXResourceReader how to depersist the object. This is currently not
     extensible. For a given mimetype the value must be set accordingly:
-    
-    Note - application/x-microsoft.net.object.binary.base64 is the format 
-    that the ResXResourceWriter will generate, however the reader can 
+
+    Note - application/x-microsoft.net.object.binary.base64 is the format
+    that the ResXResourceWriter will generate, however the reader can
     read any of the formats listed below.
-    
+
     mimetype: application/x-microsoft.net.object.binary.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Binary.BinaryFormatter
             : and then encoded with base64 encoding.
-    
+
     mimetype: application/x-microsoft.net.object.soap.base64
-    value   : The object must be serialized with 
+    value   : The object must be serialized with
             : System.Runtime.Serialization.Formatters.Soap.SoapFormatter
             : and then encoded with base64 encoding.
 
     mimetype: application/x-microsoft.net.object.bytearray.base64
-    value   : The object must be serialized into a byte array 
+    value   : The object must be serialized into a byte array
             : using a System.ComponentModel.TypeConverter
             : and then encoded with base64 encoding.
     -->
@@ -162,8 +162,8 @@
     <comment>Error when a token is not recognized</comment>
   </data>
   <data name="InvalidUserName" xml:space="preserve">
-    <value>Username '{0}' is invalid, can only contain letters or digits.</value>
-    <comment>User names can only contain letters or digits</comment>
+    <value>Username '{0}' is invalid, can only contain the following characters '{1}'.</value>
+    <comment>User names can only contain characters defined in 'AllowedUsernameCharacters'. By default these are 'a-Z0-9._@+-'</comment>
   </data>
   <data name="LoginAlreadyAssociated" xml:space="preserve">
     <value>A user with this login already exists.</value>
diff --git a/src/Identity/Extensions.Core/src/UserValidator.cs b/src/Identity/Extensions.Core/src/UserValidator.cs
index cf530106a663..1bbb9d2b3262 100644
--- a/src/Identity/Extensions.Core/src/UserValidator.cs
+++ b/src/Identity/Extensions.Core/src/UserValidator.cs
@@ -56,13 +56,13 @@ public virtual async Task<IdentityResult> ValidateAsync(UserManager<TUser> manag
         if (string.IsNullOrWhiteSpace(userName))
         {
             errors ??= new List<IdentityError>();
-            errors.Add(Describer.InvalidUserName(userName));
+            errors.Add(Describer.InvalidUserName(userName, manager.Options.User.AllowedUserNameCharacters));
         }
         else if (!string.IsNullOrEmpty(manager.Options.User.AllowedUserNameCharacters) &&
             userName.Any(c => !manager.Options.User.AllowedUserNameCharacters.Contains(c)))
         {
             errors ??= new List<IdentityError>();
-            errors.Add(Describer.InvalidUserName(userName));
+            errors.Add(Describer.InvalidUserName(userName, manager.Options.User.AllowedUserNameCharacters));
         }
         else
         {
diff --git a/src/Identity/Specification.Tests/src/UserManagerSpecificationTests.cs b/src/Identity/Specification.Tests/src/UserManagerSpecificationTests.cs
index 933dad56f92c..8ac99f5979f2 100644
--- a/src/Identity/Specification.Tests/src/UserManagerSpecificationTests.cs
+++ b/src/Identity/Specification.Tests/src/UserManagerSpecificationTests.cs
@@ -248,7 +248,7 @@ public async Task CheckSetUserNameValidatesUser()
 
         var newUser = CreateTestUser(username, useNamePrefixAsUserName: true);
         IdentityResultAssert.IsSuccess(await manager.CreateAsync(newUser));
-        var error = _errorDescriber.InvalidUserName("");
+        var error = _errorDescriber.InvalidUserName("", manager.Options.User.AllowedUserNameCharacters);
         IdentityResultAssert.IsFailure(await manager.SetUserNameAsync(newUser, ""), error);
         IdentityResultAssert.VerifyLogMessage(manager.Logger, $"User validation failed: {error.Code}.");
 
diff --git a/src/Identity/test/Identity.Test/UserValidatorTest.cs b/src/Identity/test/Identity.Test/UserValidatorTest.cs
index 30bbfd8b6a55..04d355beb778 100644
--- a/src/Identity/test/Identity.Test/UserValidatorTest.cs
+++ b/src/Identity/test/Identity.Test/UserValidatorTest.cs
@@ -32,7 +32,7 @@ public async Task ValidateFailsWithTooShortUserNames(string input)
         var result = await validator.ValidateAsync(manager, user);
 
         // Assert
-        IdentityResultAssert.IsFailure(result, new IdentityErrorDescriber().InvalidUserName(input));
+        IdentityResultAssert.IsFailure(result, new IdentityErrorDescriber().InvalidUserName(input, manager.Options.User.AllowedUserNameCharacters));
     }
 
     [Theory]