diff --git a/rhodecode/model/validation_schema/schemas/user_schema.py b/rhodecode/model/validation_schema/schemas/user_schema.py
--- a/rhodecode/model/validation_schema/schemas/user_schema.py
+++ b/rhodecode/model/validation_schema/schemas/user_schema.py
@@ -64,6 +64,7 @@ class ChangePasswordSchema(colander.Sche
 
 @colander.deferred
 def deferred_username_validator(node, kw):
+    old_username = kw.get('username')
 
     def name_validator(node, value):
         msg = _(
@@ -74,6 +75,11 @@ def deferred_username_validator(node, kw
         if not re.match(r'^[\w]{1}[\w\-\.]{0,254}$', value):
             raise colander.Invalid(node, msg)
 
+        if value != old_username:
+            existing_user = User.get_by_username(value, case_insensitive=True)
+            if existing_user:
+                raise colander.Invalid(node, 'Username is already taken')
+
     return name_validator