diff --git a/analytical/templatetags/google_analytics_gtag.py b/analytical/templatetags/google_analytics_gtag.py
index 8b4a2d2..ce9613a 100644
--- a/analytical/templatetags/google_analytics_gtag.py
+++ b/analytical/templatetags/google_analytics_gtag.py
@@ -10,6 +10,7 @@ from django.template import Library, Node, TemplateSyntaxError
 
 from analytical.utils import (
     disable_html,
+    get_identity,
     get_required_setting,
     is_internal_ip,
 )
@@ -22,10 +23,13 @@ SETUP_CODE = """
   function gtag(){{dataLayer.push(arguments);}}
   gtag('js', new Date());
 
+  {extra}
   gtag('config', '{property_id}');
 </script>
 """
 
+GTAG_SET_CODE = """gtag('set', {{'{key}': '{value}'}});"""
+
 register = Library()
 
 
@@ -51,8 +55,18 @@ class GoogleAnalyticsGTagNode(Node):
             "must be a string looking like 'UA-XXXXXX-Y'")
 
     def render(self, context):
+        other_fields = {}
+
+        identity = get_identity(context)
+        if identity is not None:
+            other_fields['user_id'] = identity
+
+        extra = '\n'.join([
+            GTAG_SET_CODE.format(key=key, value=value) for key, value in other_fields.items()
+        ])
         html = SETUP_CODE.format(
             property_id=self.property_id,
+            extra=extra,
         )
         if is_internal_ip(context, 'GOOGLE_ANALYTICS'):
             html = disable_html(html, 'Google Analytics')
diff --git a/analytical/tests/test_tag_google_analytics_gtag.py b/analytical/tests/test_tag_google_analytics_gtag.py
index f1eedd5..1eebe8b 100644
--- a/analytical/tests/test_tag_google_analytics_gtag.py
+++ b/analytical/tests/test_tag_google_analytics_gtag.py
@@ -2,6 +2,7 @@
 Tests for the Google Analytics template tags and filters, using the new gtag.js library.
 """
 
+from django.contrib.auth.models import User
 from django.http import HttpRequest
 from django.template import Context
 from django.test.utils import override_settings
@@ -50,3 +51,8 @@ class GoogleAnalyticsTagTestCase(TagTestCase):
         self.assertTrue(r.startswith(
             '<!-- Google Analytics disabled on internal IP address'), r)
         self.assertTrue(r.endswith('-->'), r)
+
+    @override_settings(ANALYTICAL_AUTO_IDENTIFY=True)
+    def test_identify(self):
+        r = GoogleAnalyticsGTagNode().render(Context({'user': User(username='test')}))
+        self.assertTrue("gtag('set', {'user_id': 'test'});" in r, r)
diff --git a/docs/services/google_analytics_gtag.rst b/docs/services/google_analytics_gtag.rst
index 68fb306..5a96a8b 100644
--- a/docs/services/google_analytics_gtag.rst
+++ b/docs/services/google_analytics_gtag.rst
@@ -81,3 +81,11 @@ setting, the tracking code is commented out.  It takes the value of
 :const:`ANALYTICAL_INTERNAL_IPS` by default (which in turn is
 :const:`INTERNAL_IPS` by default).  See :ref:`identifying-visitors` for
 important information about detecting the visitor IP address.
+
+.. _google-analytics-identify-user:
+
+Identifying authenticated users
+-------------------------------
+
+The username of an authenticated user is passed to Google Analytics
+automatically as the `user_id`.  See :ref:`identifying-visitors`.