diff --git a/changelog.d/7240.bugfix b/changelog.d/7240.bugfix
new file mode 100644
index 000000000..83b18d3e1
--- /dev/null
+++ b/changelog.d/7240.bugfix
@@ -0,0 +1 @@
+Do not allow a deactivated user to login via SSO.
diff --git a/synapse/config/sso.py b/synapse/config/sso.py
index ec3dca9ef..686678a3b 100644
--- a/synapse/config/sso.py
+++ b/synapse/config/sso.py
@@ -12,6 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import os
 from typing import Any, Dict
 
 import pkg_resources
@@ -36,6 +37,12 @@ class SSOConfig(Config):
             template_dir = pkg_resources.resource_filename("synapse", "res/templates",)
 
         self.sso_redirect_confirm_template_dir = template_dir
+        self.sso_account_deactivated_template = self.read_file(
+            os.path.join(
+                self.sso_redirect_confirm_template_dir, "sso_account_deactivated.html"
+            ),
+            "sso_account_deactivated_template",
+        )
 
         self.sso_client_whitelist = sso_config.get("client_whitelist") or []
 
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 892adb00b..fbfbd44a2 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -161,6 +161,9 @@ class AuthHandler(BaseHandler):
         self._sso_auth_confirm_template = load_jinja2_templates(
             hs.config.sso_redirect_confirm_template_dir, ["sso_auth_confirm.html"],
         )[0]
+        self._sso_account_deactivated_template = (
+            hs.config.sso_account_deactivated_template
+        )
 
         self._server_name = hs.config.server_name
 
@@ -644,9 +647,6 @@ class AuthHandler(BaseHandler):
         Returns:
             defer.Deferred: (unicode) canonical_user_id, or None if zero or
             multiple matches
-
-        Raises:
-            UserDeactivatedError if a user is found but is deactivated.
         """
         res = yield self._find_user_id_and_pwd_hash(user_id)
         if res is not None:
@@ -1099,7 +1099,7 @@ class AuthHandler(BaseHandler):
         request.write(html_bytes)
         finish_request(request)
 
-    def complete_sso_login(
+    async def complete_sso_login(
         self,
         registered_user_id: str,
         request: SynapseRequest,
@@ -1113,6 +1113,32 @@ class AuthHandler(BaseHandler):
             client_redirect_url: The URL to which to redirect the user at the end of the
                 process.
         """
+        # If the account has been deactivated, do not proceed with the login
+        # flow.
+        deactivated = await self.store.get_user_deactivated_status(registered_user_id)
+        if deactivated:
+            html = self._sso_account_deactivated_template.encode("utf-8")
+
+            request.setResponseCode(403)
+            request.setHeader(b"Content-Type", b"text/html; charset=utf-8")
+            request.setHeader(b"Content-Length", b"%d" % (len(html),))
+            request.write(html)
+            finish_request(request)
+            return
+
+        self._complete_sso_login(registered_user_id, request, client_redirect_url)
+
+    def _complete_sso_login(
+        self,
+        registered_user_id: str,
+        request: SynapseRequest,
+        client_redirect_url: str,
+    ):
+        """
+        The synchronous portion of complete_sso_login.
+
+        This exists purely for backwards compatibility of synapse.module_api.ModuleApi.
+        """
         # Create a login token
         login_token = self.macaroon_gen.generate_short_term_login_token(
             registered_user_id
diff --git a/synapse/handlers/cas_handler.py b/synapse/handlers/cas_handler.py
index d977badf3..5cb3f9d13 100644
--- a/synapse/handlers/cas_handler.py
+++ b/synapse/handlers/cas_handler.py
@@ -216,6 +216,6 @@ class CasHandler:
                     localpart=localpart, default_display_name=user_display_name
                 )
 
-            self._auth_handler.complete_sso_login(
+            await self._auth_handler.complete_sso_login(
                 registered_user_id, request, client_redirect_url
             )
diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index 4741c82f6..7c9454b50 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -154,7 +154,7 @@ class SamlHandler:
             )
 
         else:
-            self._auth_handler.complete_sso_login(user_id, request, relay_state)
+            await self._auth_handler.complete_sso_login(user_id, request, relay_state)
 
     async def _map_saml_response_to_user(
         self, resp_bytes: str, client_redirect_url: str
diff --git a/synapse/module_api/__init__.py b/synapse/module_api/__init__.py
index c7fffd72f..afc3598e1 100644
--- a/synapse/module_api/__init__.py
+++ b/synapse/module_api/__init__.py
@@ -220,6 +220,8 @@ class ModuleApi(object):
         want their access token sent to `client_redirect_url`, or redirect them to that
         URL with a token directly if the URL matches with one of the whitelisted clients.
 
+        This is deprecated in favor of complete_sso_login_async.
+
         Args:
             registered_user_id: The MXID that has been registered as a previous step of
                 of this SSO login.
@@ -227,6 +229,24 @@ class ModuleApi(object):
             client_redirect_url: The URL to which to offer to redirect the user (or to
                 redirect them directly if whitelisted).
         """
-        self._auth_handler.complete_sso_login(
+        self._auth_handler._complete_sso_login(
+            registered_user_id, request, client_redirect_url,
+        )
+
+    async def complete_sso_login_async(
+        self, registered_user_id: str, request: SynapseRequest, client_redirect_url: str
+    ):
+        """Complete a SSO login by redirecting the user to a page to confirm whether they
+        want their access token sent to `client_redirect_url`, or redirect them to that
+        URL with a token directly if the URL matches with one of the whitelisted clients.
+
+        Args:
+            registered_user_id: The MXID that has been registered as a previous step of
+                of this SSO login.
+            request: The request to respond to.
+            client_redirect_url: The URL to which to offer to redirect the user (or to
+                redirect them directly if whitelisted).
+        """
+        await self._auth_handler.complete_sso_login(
             registered_user_id, request, client_redirect_url,
         )
diff --git a/synapse/res/templates/sso_account_deactivated.html b/synapse/res/templates/sso_account_deactivated.html
new file mode 100644
index 000000000..4eb8db9fb
--- /dev/null
+++ b/synapse/res/templates/sso_account_deactivated.html
@@ -0,0 +1,10 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <title>SSO account deactivated</title>
+</head>
+    <body>
+        <p>This account has been deactivated.</p>
+    </body>
+</html>
diff --git a/tests/rest/client/v1/test_login.py b/tests/rest/client/v1/test_login.py
index aed8853d6..1856c7ffd 100644
--- a/tests/rest/client/v1/test_login.py
+++ b/tests/rest/client/v1/test_login.py
@@ -257,7 +257,7 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
         self.assertEquals(channel.code, 200, channel.result)
 
 
-class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
+class CASTestCase(unittest.HomeserverTestCase):
 
     servlets = [
         login.register_servlets,
@@ -274,6 +274,9 @@ class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
             "service_url": "https://matrix.goodserver.com:8448",
         }
 
+        cas_user_id = "username"
+        self.user_id = "@%s:test" % cas_user_id
+
         async def get_raw(uri, args):
             """Return an example response payload from a call to the `/proxyValidate`
             endpoint of a CAS server, copied from
@@ -282,10 +285,11 @@ class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
             This needs to be returned by an async function (as opposed to set as the
             mock's return value) because the corresponding Synapse code awaits on it.
             """
-            return """
+            return (
+                """
                 <cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
                   <cas:authenticationSuccess>
-                      <cas:user>username</cas:user>
+                      <cas:user>%s</cas:user>
                       <cas:proxyGrantingTicket>PGTIOU-84678-8a9d...</cas:proxyGrantingTicket>
                       <cas:proxies>
                           <cas:proxy>https://proxy2/pgtUrl</cas:proxy>
@@ -294,6 +298,8 @@ class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
                   </cas:authenticationSuccess>
                 </cas:serviceResponse>
             """
+                % cas_user_id
+            )
 
         mocked_http_client = Mock(spec=["get_raw"])
         mocked_http_client.get_raw.side_effect = get_raw
@@ -304,6 +310,9 @@ class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
 
         return self.hs
 
+    def prepare(self, reactor, clock, hs):
+        self.deactivate_account_handler = hs.get_deactivate_account_handler()
+
     def test_cas_redirect_confirm(self):
         """Tests that the SSO login flow serves a confirmation page before redirecting a
         user to the redirect URL.
@@ -370,3 +379,30 @@ class CASRedirectConfirmTestCase(unittest.HomeserverTestCase):
         self.assertEqual(channel.code, 302)
         location_headers = channel.headers.getRawHeaders("Location")
         self.assertEqual(location_headers[0][: len(redirect_url)], redirect_url)
+
+    @override_config({"sso": {"client_whitelist": ["https://legit-site.com/"]}})
+    def test_deactivated_user(self):
+        """Logging in as a deactivated account should error."""
+        redirect_url = "https://legit-site.com/"
+
+        # First login (to create the user).
+        self._test_redirect(redirect_url)
+
+        # Deactivate the account.
+        self.get_success(
+            self.deactivate_account_handler.deactivate_account(self.user_id, False)
+        )
+
+        # Request the CAS ticket.
+        cas_ticket_url = (
+            "/_matrix/client/r0/login/cas/ticket?redirectUrl=%s&ticket=ticket"
+            % (urllib.parse.quote(redirect_url))
+        )
+
+        # Get Synapse to call the fake CAS and serve the template.
+        request, channel = self.make_request("GET", cas_ticket_url)
+        self.render(request)
+
+        # Because the user is deactivated they are served an error template.
+        self.assertEqual(channel.code, 403)
+        self.assertIn(b"SSO account deactivated", channel.result["body"])