diff --git a/changelog.d/9091.feature b/changelog.d/9091.feature
new file mode 100644
index 000000000..79fcd701f
--- /dev/null
+++ b/changelog.d/9091.feature
@@ -0,0 +1 @@
+During user-interactive authentication via single-sign-on, give a better error if the user uses the wrong account on the SSO IdP.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index c8ae46d1b..9da351f9f 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -1969,6 +1969,14 @@ sso:
     #
     #   This template has no additional variables.
     #
+    # * HTML page shown after a user-interactive authentication session which
+    #   does not map correctly onto the expected user: 'sso_auth_bad_user.html'.
+    #
+    #   When rendering, this template is given the following variables:
+    #     * server_name: the homeserver's name.
+    #     * user_id_to_verify: the MXID of the user that we are trying to
+    #       validate.
+    #
     # * HTML page shown during single sign-on if a deactivated user (according to Synapse's database)
     #   attempts to login: 'sso_account_deactivated.html'.
     #
diff --git a/synapse/config/sso.py b/synapse/config/sso.py
index 1aeb1c5c9..366f0d469 100644
--- a/synapse/config/sso.py
+++ b/synapse/config/sso.py
@@ -37,6 +37,7 @@ class SSOConfig(Config):
             self.sso_error_template,
             sso_account_deactivated_template,
             sso_auth_success_template,
+            self.sso_auth_bad_user_template,
         ) = self.read_templates(
             [
                 "sso_login_idp_picker.html",
@@ -45,6 +46,7 @@ class SSOConfig(Config):
                 "sso_error.html",
                 "sso_account_deactivated.html",
                 "sso_auth_success.html",
+                "sso_auth_bad_user.html",
             ],
             template_dir,
         )
@@ -160,6 +162,14 @@ class SSOConfig(Config):
             #
             #   This template has no additional variables.
             #
+            # * HTML page shown after a user-interactive authentication session which
+            #   does not map correctly onto the expected user: 'sso_auth_bad_user.html'.
+            #
+            #   When rendering, this template is given the following variables:
+            #     * server_name: the homeserver's name.
+            #     * user_id_to_verify: the MXID of the user that we are trying to
+            #       validate.
+            #
             # * HTML page shown during single sign-on if a deactivated user (according to Synapse's database)
             #   attempts to login: 'sso_account_deactivated.html'.
             #
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index d096e0b09..69ffc9d9c 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -23,6 +23,7 @@ from typing_extensions import NoReturn, Protocol
 from twisted.web.http import Request
 
 from synapse.api.errors import Codes, RedirectException, SynapseError
+from synapse.handlers.ui_auth import UIAuthSessionDataConstants
 from synapse.http import get_request_user_agent
 from synapse.http.server import respond_with_html
 from synapse.http.site import SynapseRequest
@@ -147,6 +148,7 @@ class SsoHandler:
         self._server_name = hs.hostname
         self._registration_handler = hs.get_registration_handler()
         self._error_template = hs.config.sso_error_template
+        self._bad_user_template = hs.config.sso_auth_bad_user_template
         self._auth_handler = hs.get_auth_handler()
 
         # a lock on the mappings
@@ -577,20 +579,41 @@ class SsoHandler:
             auth_provider_id, remote_user_id,
         )
 
+        user_id_to_verify = await self._auth_handler.get_session_data(
+            ui_auth_session_id, UIAuthSessionDataConstants.REQUEST_USER_ID
+        )  # type: str
+
         if not user_id:
             logger.warning(
                 "Remote user %s/%s has not previously logged in here: UIA will fail",
                 auth_provider_id,
                 remote_user_id,
             )
-            # Let the UIA flow handle this the same as if they presented creds for a
-            # different user.
-            user_id = ""
+        elif user_id != user_id_to_verify:
+            logger.warning(
+                "Remote user %s/%s mapped onto incorrect user %s: UIA will fail",
+                auth_provider_id,
+                remote_user_id,
+                user_id,
+            )
+        else:
+            # success!
+            await self._auth_handler.complete_sso_ui_auth(
+                user_id, ui_auth_session_id, request
+            )
+            return
 
-        await self._auth_handler.complete_sso_ui_auth(
-            user_id, ui_auth_session_id, request
+        # the user_id didn't match: mark the stage of the authentication as unsuccessful
+        await self._store.mark_ui_auth_stage_complete(
+            ui_auth_session_id, LoginType.SSO, ""
         )
 
+        # render an error page.
+        html = self._bad_user_template.render(
+            server_name=self._server_name, user_id_to_verify=user_id_to_verify,
+        )
+        respond_with_html(request, 200, html)
+
     async def check_username_availability(
         self, localpart: str, session_id: str,
     ) -> bool:
diff --git a/synapse/res/templates/sso_auth_bad_user.html b/synapse/res/templates/sso_auth_bad_user.html
new file mode 100644
index 000000000..3611191bf
--- /dev/null
+++ b/synapse/res/templates/sso_auth_bad_user.html
@@ -0,0 +1,18 @@
+<html>
+<head>
+    <title>Authentication Failed</title>
+</head>
+    <body>
+        <div>
+            <p>
+                We were unable to validate your <tt>{{server_name | e}}</tt> account via
+                single-sign-on (SSO), because the SSO Identity Provider returned
+                different details than when you logged in.
+            </p>
+            <p>
+                Try the operation again, and ensure that you use the same details on
+                the Identity Provider as when you log into your account.
+            </p>
+        </div>
+    </body>
+</html>