diff --git a/docs/overview/changelog.md b/docs/overview/changelog.md
index f42dc8ea07..79abc03974 100644
--- a/docs/overview/changelog.md
+++ b/docs/overview/changelog.md
@@ -20,6 +20,9 @@ in bursts.
 - Added an organization setting to control who can edit topics.
 - Added ctrl+K keyboard shortcut for getting to search (same as /, but
   works even when you're inside compose).
+- Added the new `SOCIAL_AUTH_SUBDOMAIN` setting, which all servers using
+  both GitHub authentication and hosting multiple Zulip organizations
+  should set (see [the docs for details](../production/multiple-organizations.html#social-authentication)).
 - Optimized the performance of loading Zulip in an organization with
   thousands of users and hundreds of bot users.
 - Removed the "Delete streams" administration page; one can delete
diff --git a/docs/production/multiple-organizations.md b/docs/production/multiple-organizations.md
index 4aaf94c62b..e38677f492 100644
--- a/docs/production/multiple-organizations.md
+++ b/docs/production/multiple-organizations.md
@@ -41,6 +41,9 @@ things:
   new organization.  Review
   [the install instructions](install.html) if you need a
   refresher on how this works.
+* If you're planning on using GitHub auth or another social
+  authentication method, review
+  [the notes on `SOCIAL_AUTH_SUBDOMAIN` below](#social-authentication).
 
 For servers hosting a large number of organizations, like
 [zulipchat.com](https://zulipchat.com), one can set
@@ -80,6 +83,17 @@ visible to the subdomain (so it's not possible for a single
 browser/client to be logged into both).  So we don't recommend that
 configuration.
 
+### Social authentication
+
+If you're using GitHub authentication (or any other authentication
+backend that we implement using python-social-auth), you will likely
+want to set the `SOCIAL_AUTH_SUBDOMAIN` setting to something (`'auth'`
+is a good choice) and update the GitHub authentication callback URL to
+be that subdomain.  Otherwise, your users will experience confusing
+behavior where attempting to login using a social authentication
+backend will appear to log them out of the other organizations on your
+server.
+
 ### The system bot realm
 
 This is very much an implementation detail, but worth documenting to
diff --git a/zerver/lib/name_restrictions.py b/zerver/lib/name_restrictions.py
index 69561bd7e5..20fc36db9c 100644
--- a/zerver/lib/name_restrictions.py
+++ b/zerver/lib/name_restrictions.py
@@ -34,6 +34,8 @@ ZULIP_RESERVED_SUBDOMAINS = frozenset([
     'contribute', 'floss', 'foss', 'free', 'opensource', 'open', 'code', 'license',
     # intership programs
     'intern', 'outreachy', 'gsoc', 'gci', 'externship',
+    # Things that sound like security
+    'auth', 'authentication', 'security',
     # tech blogs
     'engineering', 'infrastructure', 'tooling', 'tools', 'javascript', 'python'])
 
diff --git a/zerver/tests/test_auth_backends.py b/zerver/tests/test_auth_backends.py
index 745a6eaf07..5042a6de6f 100644
--- a/zerver/tests/test_auth_backends.py
+++ b/zerver/tests/test_auth_backends.py
@@ -443,7 +443,13 @@ class GitHubAuthBackendTest(ZulipTestCase):
             url += "?%s" % (urllib.parse.urlencode(params))
 
         result = self.client_get(url, **headers)
-        if result.status_code != 302 or 'http://testserver/login/github/' not in result.url:
+
+        expected_result_url_prefix = 'http://testserver/login/github/'
+        if settings.SOCIAL_AUTH_SUBDOMAIN is not None:
+            expected_result_url_prefix = ('http://%s.testserver/login/github/' %
+                                          settings.SOCIAL_AUTH_SUBDOMAIN)
+
+        if result.status_code != 302 or not result.url.startswith(expected_result_url_prefix):
             return result
 
         result = self.client_get(result.url, **headers)
@@ -527,6 +533,22 @@ class GitHubAuthBackendTest(ZulipTestCase):
                                  parsed_url.path)
         self.assertTrue(uri.startswith('http://zulip.testserver/accounts/login/subdomain/'))
 
+    @override_settings(SOCIAL_AUTH_SUBDOMAIN=None)
+    def test_github_when_social_auth_subdomain_is_not_set(self) -> None:
+        account_data_dict = dict(email=self.email, name=self.name)
+        result = self.github_oauth2_test(account_data_dict,
+                                         subdomain='zulip', next='/user_uploads/image')
+        data = load_subdomain_token(result)
+        self.assertEqual(data['email'], self.example_email("hamlet"))
+        self.assertEqual(data['name'], 'Hamlet')
+        self.assertEqual(data['subdomain'], 'zulip')
+        self.assertEqual(data['next'], '/user_uploads/image')
+        self.assertEqual(result.status_code, 302)
+        parsed_url = urllib.parse.urlparse(result.url)
+        uri = "{}://{}{}".format(parsed_url.scheme, parsed_url.netloc,
+                                 parsed_url.path)
+        self.assertTrue(uri.startswith('http://zulip.testserver/accounts/login/subdomain/'))
+
     def test_github_oauth2_email_not_verified(self) -> None:
         account_data_dict = dict(email=self.email, name=self.name)
         with mock.patch('logging.warning') as mock_warning:
diff --git a/zerver/views/auth.py b/zerver/views/auth.py
index 6a492cc649..2d053ccdc8 100644
--- a/zerver/views/auth.py
+++ b/zerver/views/auth.py
@@ -275,8 +275,15 @@ def google_oauth2_csrf(request: HttpRequest, value: str) -> str:
 def reverse_on_root(viewname: str, args: List[str]=None, kwargs: Dict[str, str]=None) -> str:
     return settings.ROOT_DOMAIN_URI + reverse(viewname, args=args, kwargs=kwargs)
 
-def oauth_redirect_to_root(request: HttpRequest, url: str, is_signup: bool=False) -> HttpResponse:
+def oauth_redirect_to_root(request: HttpRequest, url: str,
+                           sso_type: str, is_signup: bool=False) -> HttpResponse:
     main_site_uri = settings.ROOT_DOMAIN_URI + url
+    if settings.SOCIAL_AUTH_SUBDOMAIN is not None and sso_type == 'social':
+        main_site_uri = (settings.EXTERNAL_URI_SCHEME +
+                         settings.SOCIAL_AUTH_SUBDOMAIN +
+                         "." +
+                         settings.EXTERNAL_HOST) + url
+
     params = {
         'subdomain': get_subdomain(request),
         'is_signup': '1' if is_signup else '0',
@@ -303,7 +310,7 @@ def start_google_oauth2(request: HttpRequest) -> HttpResponse:
         return redirect_to_config_error("google")
 
     is_signup = bool(request.GET.get('is_signup'))
-    return oauth_redirect_to_root(request, url, is_signup=is_signup)
+    return oauth_redirect_to_root(request, url, 'google', is_signup=is_signup)
 
 def start_social_login(request: HttpRequest, backend: str) -> HttpResponse:
     backend_url = reverse('social:begin', args=[backend])
@@ -311,11 +318,11 @@ def start_social_login(request: HttpRequest, backend: str) -> HttpResponse:
                                       settings.SOCIAL_AUTH_GITHUB_SECRET):
         return redirect_to_config_error("github")
 
-    return oauth_redirect_to_root(request, backend_url)
+    return oauth_redirect_to_root(request, backend_url, 'social')
 
 def start_social_signup(request: HttpRequest, backend: str) -> HttpResponse:
     backend_url = reverse('social:begin', args=[backend])
-    return oauth_redirect_to_root(request, backend_url, is_signup=True)
+    return oauth_redirect_to_root(request, backend_url, 'social', is_signup=True)
 
 def send_oauth_request_to_google(request: HttpRequest) -> HttpResponse:
     subdomain = request.GET.get('subdomain', '')
diff --git a/zproject/prod_settings_template.py b/zproject/prod_settings_template.py
index 1d9f333525..b550da8fbc 100644
--- a/zproject/prod_settings_template.py
+++ b/zproject/prod_settings_template.py
@@ -146,7 +146,7 @@ AUTHENTICATION_BACKENDS = (
 #   https://github.com/organizations/ORGNAME/settings/developers
 # Fill in "Callback URL" with a value like
 #   https://zulip.example.com/complete/github/ as
-# based on your value for EXTERNAL_HOST.
+# based on your values for EXTERNAL_HOST and SOCIAL_AUTH_SUBDOMAIN.
 #
 # (2) You should get a page with settings for your new application,
 # showing a client ID and a client secret.  Use the client ID as
@@ -161,6 +161,15 @@ AUTHENTICATION_BACKENDS = (
 #SOCIAL_AUTH_GITHUB_TEAM_ID = <your team id>
 #SOCIAL_AUTH_GITHUB_ORG_NAME = <your org name>
 
+# (4) If you are serving multiple Zulip organizations on different
+# subdomains, you need to set SOCIAL_AUTH_SUBDOMAIN.  You can set it
+# to any subdomain on which you do not plan to host a Zulip
+# organization.  The default recommendation, `auth`, is a reserved
+# subdomain; if you're using this setting, the "Callback URL" should be e.g.:
+#   https://auth.zulip.example.com/complete/github/
+#
+#SOCIAL_AUTH_SUBDOMAIN = 'auth'
+
 ########
 # SSO via REMOTE_USER.
 #
diff --git a/zproject/settings.py b/zproject/settings.py
index 192c715f36..1e12064eb4 100644
--- a/zproject/settings.py
+++ b/zproject/settings.py
@@ -145,6 +145,7 @@ DEFAULT_SETTINGS = {
     'SOCIAL_AUTH_GITHUB_KEY': get_secret('social_auth_github_key', development_only=True),
     'SOCIAL_AUTH_GITHUB_ORG_NAME': None,
     'SOCIAL_AUTH_GITHUB_TEAM_ID': None,
+    'SOCIAL_AUTH_SUBDOMAIN': None,
 
     # Email gateway
     'EMAIL_GATEWAY_PATTERN': '',
diff --git a/zproject/test_settings.py b/zproject/test_settings.py
index b6070598fa..1e3b7a0820 100644
--- a/zproject/test_settings.py
+++ b/zproject/test_settings.py
@@ -145,6 +145,7 @@ GOOGLE_OAUTH2_CLIENT_SECRET = "secret"
 
 SOCIAL_AUTH_GITHUB_KEY = "key"
 SOCIAL_AUTH_GITHUB_SECRET = "secret"
+SOCIAL_AUTH_SUBDOMAIN = 'www'
 
 # By default two factor authentication is disabled in tests.
 # Explicitly set this to True within tests that must have this on.