diff --git a/django/contrib/auth/migrations/0008_alter_user_username_max_length.py b/django/contrib/auth/migrations/0008_alter_user_username_max_length.py
index 56705fa58f..2afbc30c2a 100644
--- a/django/contrib/auth/migrations/0008_alter_user_username_max_length.py
+++ b/django/contrib/auth/migrations/0008_alter_user_username_max_length.py
@@ -17,8 +17,8 @@ class Migration(migrations.Migration):
             name='username',
             field=models.CharField(
                 error_messages={'unique': 'A user with that username already exists.'},
-                help_text='Required. 254 characters or fewer. Letters, digits and @/./+/-/_ only.',
-                max_length=254,
+                help_text='Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.',
+                max_length=150,
                 unique=True,
                 validators=[
                     django.core.validators.RegexValidator(
diff --git a/django/contrib/auth/models.py b/django/contrib/auth/models.py
index dfa76977d7..0600b96774 100644
--- a/django/contrib/auth/models.py
+++ b/django/contrib/auth/models.py
@@ -303,9 +303,9 @@ class AbstractUser(AbstractBaseUser, PermissionsMixin):
     """
     username = models.CharField(
         _('username'),
-        max_length=254,
+        max_length=150,
         unique=True,
-        help_text=_('Required. 254 characters or fewer. Letters, digits and @/./+/-/_ only.'),
+        help_text=_('Required. 150 characters or fewer. Letters, digits and @/./+/-/_ only.'),
         validators=[
             validators.RegexValidator(
                 r'^[\w.@+-]+$',
diff --git a/docs/ref/contrib/auth.txt b/docs/ref/contrib/auth.txt
index 6ca2581397..e4c31731d5 100644
--- a/docs/ref/contrib/auth.txt
+++ b/docs/ref/contrib/auth.txt
@@ -21,12 +21,19 @@ Fields
 
     .. attribute:: username
 
-        Required. 254 characters or fewer. Usernames may contain alphanumeric,
+        Required. 150 characters or fewer. Usernames may contain alphanumeric,
         ``_``, ``@``, ``+``, ``.`` and ``-`` characters.
 
+        The ``max_length`` should be sufficient for many use cases. If you need
+        a longer length, please use a :ref:`custom user model
+        <specifying-custom-user-model>`. If you use MySQL with the ``utf8mb4``
+        encoding (recommended for proper Unicode support), specify at most
+        ``max_length=191`` because MySQL can only create unique indexes with
+        191 characters in that case by default.
+
         .. versionchanged:: 1.10
 
-            The ``max_length`` increased from 30 to 254 characters.
+            The ``max_length`` increased from 30 to 150 characters.
 
     .. attribute:: first_name
 
diff --git a/docs/releases/1.10.txt b/docs/releases/1.10.txt
index cf9b67d2d0..f3c7669d8a 100644
--- a/docs/releases/1.10.txt
+++ b/docs/releases/1.10.txt
@@ -354,13 +354,20 @@ to its proxied concrete class. This inconsistency was fixed by returning the
 full set of fields pointing to a concrete class or one of its proxies in both
 cases.
 
-:attr:`AbstractUser.username <django.contrib.auth.models.User.username>` ``max_length`` increased to 254
+:attr:`AbstractUser.username <django.contrib.auth.models.User.username>` ``max_length`` increased to 150
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 A migration for :attr:`django.contrib.auth.models.User.username` is included.
 If you have a custom user model inheriting from ``AbstractUser``, you'll need
 to generate and apply a database migration for your user model.
 
+We considered an increase to 254 characters to more easily allow the use of
+email addresses (which are limited to 254 characters) as usernames but rejected
+it due to a MySQL limitation.  When using the ``utf8mb4`` encoding (recommended
+for proper Unicode support), MySQL can only create unique indexes with 191
+characters by default. Therefore, if you need a longer length, please use a
+custom user model.
+
 If you want to preserve the 30 character limit for usernames, use a custom form
 when creating a user or changing usernames::