diff --git a/apps/server/AliasVault.Admin/Main/Pages/Users/View/Index.razor b/apps/server/AliasVault.Admin/Main/Pages/Users/View/Index.razor
index b070456c6..e7a9317ca 100644
--- a/apps/server/AliasVault.Admin/Main/Pages/Users/View/Index.razor
+++ b/apps/server/AliasVault.Admin/Main/Pages/Users/View/Index.razor
@@ -2,6 +2,7 @@
 @using AliasVault.Admin.Main.Pages.Users.View.Components
 @using AliasVault.Admin.Main.Models
 @using AliasVault.Admin.Services
+@using AliasVault.Auth
 @using Microsoft.EntityFrameworkCore
 @inherits MainBase
 @inject StatisticsService StatisticsService
@@ -642,6 +643,9 @@ Do you want to proceed with the restoration?")) {
     /// </summary>
     private async Task ChangeUsername()
     {
+        // Normalize to lowercase and trim to match how usernames are stored at registration.
+        NewUsername = UsernameHelper.NormalizeUsername(NewUsername);
+
         if (string.IsNullOrWhiteSpace(NewUsername))
         {
             UsernameValidationError = "Username cannot be empty.";
diff --git a/apps/server/AliasVault.Api/Controllers/AuthController.cs b/apps/server/AliasVault.Api/Controllers/AuthController.cs
index 5f51b1a1e..dd4111951 100644
--- a/apps/server/AliasVault.Api/Controllers/AuthController.cs
+++ b/apps/server/AliasVault.Api/Controllers/AuthController.cs
@@ -458,7 +458,7 @@ public class AuthController(IAliasServerDbContextFactory dbContextFactory, UserM
 
         var user = new AliasVaultUser
         {
-            UserName = model.Username,
+            UserName = UsernameHelper.NormalizeUsername(model.Username),
             SrpIdentity = srpIdentity,
             CreatedAt = timeProvider.UtcNow,
             UpdatedAt = timeProvider.UtcNow,
@@ -551,7 +551,7 @@ public class AuthController(IAliasServerDbContextFactory dbContextFactory, UserM
             return BadRequest(ApiErrorCodeHelper.CreateErrorResponse(ApiErrorCode.USERNAME_REQUIRED, 400));
         }
 
-        var normalizedUsername = NormalizeUsername(model.Username);
+        var normalizedUsername = UsernameHelper.NormalizeUsername(model.Username);
         var existingUser = await userManager.FindByNameAsync(normalizedUsername);
 
         if (existingUser != null)
@@ -586,7 +586,7 @@ public class AuthController(IAliasServerDbContextFactory dbContextFactory, UserM
         }
 
         // Verify the username matches the current user.
-        if (user.UserName != model.Username)
+        if (!string.Equals(user.UserName, model.Username, StringComparison.OrdinalIgnoreCase))
         {
             return BadRequest(ApiErrorCodeHelper.CreateValidationErrorResponse(ApiErrorCode.USERNAME_MISMATCH, 400));
         }
@@ -826,7 +826,7 @@ public class AuthController(IAliasServerDbContextFactory dbContextFactory, UserM
         }
 
         // Verify the username matches the current user.
-        if (user.UserName != model.Username)
+        if (!string.Equals(user.UserName, model.Username, StringComparison.OrdinalIgnoreCase))
         {
             return BadRequest(ApiErrorCodeHelper.CreateValidationErrorResponse(ApiErrorCode.USERNAME_MISMATCH, 400));
         }
@@ -850,16 +850,6 @@ public class AuthController(IAliasServerDbContextFactory dbContextFactory, UserM
         return Ok(ApiErrorCodeHelper.CreateErrorResponse(ApiErrorCode.ACCOUNT_SUCCESSFULLY_DELETED, 200));
     }
 
-    /// <summary>
-    /// Normalizes a username by trimming and lowercasing it.
-    /// </summary>
-    /// <param name="username">The username to normalize.</param>
-    /// <returns>The normalized username.</returns>
-    private static string NormalizeUsername(string username)
-    {
-        return username.ToLowerInvariant().Trim();
-    }
-
     /// <summary>
     /// Generate a refresh token for a user. This token is used to request a new access token when the current
     /// access token expires. The refresh token is long-lived by design.
diff --git a/apps/server/AliasVault.Api/Controllers/VaultController.cs b/apps/server/AliasVault.Api/Controllers/VaultController.cs
index a36cd63a6..1da925eaa 100644
--- a/apps/server/AliasVault.Api/Controllers/VaultController.cs
+++ b/apps/server/AliasVault.Api/Controllers/VaultController.cs
@@ -145,7 +145,7 @@ public class VaultController(ILogger<VaultController> logger, IAliasServerDbCont
         // If they do not match reject the request. This is important because it's
         // possible that a user has logged in with a different username than the one
         // that is being used to update the vault (e.g. if working with multiple tabs).
-        if (user.UserName != model.Username)
+        if (!string.Equals(user.UserName, model.Username, StringComparison.OrdinalIgnoreCase))
         {
             return BadRequest(ApiErrorCodeHelper.CreateValidationErrorResponse(ApiErrorCode.USERNAME_MISMATCH, 400));
         }
@@ -233,7 +233,7 @@ public class VaultController(ILogger<VaultController> logger, IAliasServerDbCont
         // If they do not match reject the request. This is important because it's
         // possible that a user has logged in with a different username than the one
         // that is being used to update the vault (e.g. if working with multiple tabs).
-        if (model.Username != user.UserName)
+        if (!string.Equals(user.UserName, model.Username, StringComparison.OrdinalIgnoreCase))
         {
             return BadRequest(ApiErrorCodeHelper.CreateValidationErrorResponse(ApiErrorCode.USERNAME_MISMATCH, 400));
         }
diff --git a/apps/server/Utilities/AliasVault.Auth/UsernameHelper.cs b/apps/server/Utilities/AliasVault.Auth/UsernameHelper.cs
new file mode 100644
index 000000000..101d3f492
--- /dev/null
+++ b/apps/server/Utilities/AliasVault.Auth/UsernameHelper.cs
@@ -0,0 +1,25 @@
+//-----------------------------------------------------------------------
+// <copyright file="UsernameHelper.cs" company="aliasvault">
+// Copyright (c) aliasvault. All rights reserved.
+// Licensed under the AGPLv3 license. See LICENSE.md file in the project root for full license information.
+// </copyright>
+//-----------------------------------------------------------------------
+
+namespace AliasVault.Auth;
+
+/// <summary>
+/// Helper for normalizing usernames consistently across the server.
+/// </summary>
+public static class UsernameHelper
+{
+    /// <summary>
+    /// Normalizes a username by lowercasing and trimming it. 
+    /// Used by all code paths that store a username.
+    /// </summary>
+    /// <param name="username">The username to normalize.</param>
+    /// <returns>The normalized username.</returns>
+    public static string NormalizeUsername(string username)
+    {
+        return username.ToLowerInvariant().Trim();
+    }
+}