diff --git a/changelog.d/12210.misc b/changelog.d/12210.misc
new file mode 100644
index 000000000000..3f6a8747c256
--- /dev/null
+++ b/changelog.d/12210.misc
@@ -0,0 +1 @@
+Update the SSO username picker template to comply with SIWA guidelines.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 6f3623c88ab9..ef25a3175f98 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -1947,8 +1947,13 @@ saml2_config:
 #
 #             localpart_template: Jinja2 template for the localpart of the MXID.
 #                 If this is not set, the user will be prompted to choose their
-#                 own username (see 'sso_auth_account_details.html' in the 'sso'
-#                 section of this file).
+#                 own username (see the documentation for the
+#                 'sso_auth_account_details.html' template).
+#
+#             confirm_localpart: Whether to prompt the user to validate (or
+#                 change) the generated localpart (see the documentation for the
+#                 'sso_auth_account_details.html' template), instead of
+#                 registering the account right away.
 #
 #             display_name_template: Jinja2 template for the display name to set
 #                 on first login. If unset, no displayname will be set.
diff --git a/docs/templates.md b/docs/templates.md
index 2b66e9d86294..b251d05cb9eb 100644
--- a/docs/templates.md
+++ b/docs/templates.md
@@ -176,8 +176,11 @@ Below are the templates Synapse will look for when generating pages related to S
              for the brand of the IdP
     * `user_attributes`: an object containing details about the user that
       we received from the IdP. May have the following attributes:
-        * display_name: the user's display_name
-        * emails: a list of email addresses
+        * `display_name`: the user's display name
+        * `emails`: a list of email addresses
+        * `localpart`: the local part of the Matrix user ID to register,
+          if `localpart_template` is set in the mapping provider configuration (empty
+          string if not)
   The template should render a form which submits the following fields:
     * `username`: the localpart of the user's chosen user id
 * `sso_new_user_consent.html`: HTML page allowing the user to consent to the
diff --git a/synapse/config/oidc.py b/synapse/config/oidc.py
index f7e4f9ef22b1..fc95912d9b3f 100644
--- a/synapse/config/oidc.py
+++ b/synapse/config/oidc.py
@@ -182,8 +182,13 @@ def generate_config_section(self, config_dir_path, server_name, **kwargs) -> str
         #
         #             localpart_template: Jinja2 template for the localpart of the MXID.
         #                 If this is not set, the user will be prompted to choose their
-        #                 own username (see 'sso_auth_account_details.html' in the 'sso'
-        #                 section of this file).
+        #                 own username (see the documentation for the
+        #                 'sso_auth_account_details.html' template).
+        #
+        #             confirm_localpart: Whether to prompt the user to validate (or
+        #                 change) the generated localpart (see the documentation for the
+        #                 'sso_auth_account_details.html' template), instead of
+        #                 registering the account right away.
         #
         #             display_name_template: Jinja2 template for the display name to set
         #                 on first login. If unset, no displayname will be set.
diff --git a/synapse/handlers/oidc.py b/synapse/handlers/oidc.py
index 593a2aac6691..d98659edc761 100644
--- a/synapse/handlers/oidc.py
+++ b/synapse/handlers/oidc.py
@@ -1228,6 +1228,7 @@ class OidcSessionData:
 
 class UserAttributeDict(TypedDict):
     localpart: Optional[str]
+    confirm_localpart: bool
     display_name: Optional[str]
     emails: List[str]
 
@@ -1316,6 +1317,7 @@ class JinjaOidcMappingConfig:
     display_name_template: Optional[Template]
     email_template: Optional[Template]
     extra_attributes: Dict[str, Template]
+    confirm_localpart: bool = False
 
 
 class JinjaOidcMappingProvider(OidcMappingProvider[JinjaOidcMappingConfig]):
@@ -1357,12 +1359,17 @@ def parse_template_config(option_name: str) -> Optional[Template]:
                         "invalid jinja template", path=["extra_attributes", key]
                     ) from e
 
+        confirm_localpart = config.get("confirm_localpart") or False
+        if not isinstance(confirm_localpart, bool):
+            raise ConfigError("must be a bool", path=["confirm_localpart"])
+
         return JinjaOidcMappingConfig(
             subject_claim=subject_claim,
             localpart_template=localpart_template,
             display_name_template=display_name_template,
             email_template=email_template,
             extra_attributes=extra_attributes,
+            confirm_localpart=confirm_localpart,
         )
 
     def get_remote_user_id(self, userinfo: UserInfo) -> str:
@@ -1398,7 +1405,10 @@ def render_template_field(template: Optional[Template]) -> Optional[str]:
             emails.append(email)
 
         return UserAttributeDict(
-            localpart=localpart, display_name=display_name, emails=emails
+            localpart=localpart,
+            display_name=display_name,
+            emails=emails,
+            confirm_localpart=self._config.confirm_localpart,
         )
 
     async def get_extra_attributes(self, userinfo: UserInfo, token: Token) -> JsonDict:
diff --git a/synapse/handlers/sso.py b/synapse/handlers/sso.py
index ff5b5169cac1..4f02a060d953 100644
--- a/synapse/handlers/sso.py
+++ b/synapse/handlers/sso.py
@@ -132,6 +132,7 @@ class UserAttributes:
     # if `None`, the mapper has not picked a userid, and the user should be prompted to
     # enter one.
     localpart: Optional[str]
+    confirm_localpart: bool = False
     display_name: Optional[str] = None
     emails: Collection[str] = attr.Factory(list)
 
@@ -561,9 +562,10 @@ def _get_url_for_next_new_user_step(
         # Must provide either attributes or session, not both
         assert (attributes is not None) != (session is not None)
 
-        if (attributes and attributes.localpart is None) or (
-            session and session.chosen_localpart is None
-        ):
+        if (
+            attributes
+            and (attributes.localpart is None or attributes.confirm_localpart is True)
+        ) or (session and session.chosen_localpart is None):
             return b"/_synapse/client/pick_username/account_details"
         elif self._consent_at_registration and not (
             session and session.terms_accepted_version
diff --git a/synapse/res/templates/sso_auth_account_details.html b/synapse/res/templates/sso_auth_account_details.html
index 00e1dcdbb866..41315e4fd4da 100644
--- a/synapse/res/templates/sso_auth_account_details.html
+++ b/synapse/res/templates/sso_auth_account_details.html
@@ -130,15 +130,15 @@
   </head>
   <body>
     <header>
-      <h1>Your account is nearly ready</h1>
-      <p>Check your details before creating an account on {{ server_name }}</p>
+      <h1>Choose your user name</h1>
+      <p>This is required to create your account on {{ server_name }}, and you can't change this later.</p>
     </header>
     <main>
       <form method="post" class="form__input" id="form">
         <div class="username_input" id="username_input">
           <label for="field-username">Username</label>
           <div class="prefix">@</div>
-          <input type="text" name="username" id="field-username" autofocus>
+          <input type="text" name="username" id="field-username" value="{{ user_attributes.localpart }}" autofocus>
           <div class="postfix">:{{ server_name }}</div>
         </div>
         <output for="username_input" id="field-username-output"></output>
diff --git a/synapse/rest/synapse/client/pick_username.py b/synapse/rest/synapse/client/pick_username.py
index 28ae08349705..6338fbaaa96d 100644
--- a/synapse/rest/synapse/client/pick_username.py
+++ b/synapse/rest/synapse/client/pick_username.py
@@ -92,12 +92,20 @@ async def _async_render_GET(self, request: Request) -> None:
             self._sso_handler.render_error(request, "bad_session", e.msg, code=e.code)
             return
 
+        # The configuration might mandate going through this step to validate an
+        # automatically generated localpart, so session.chosen_localpart might already
+        # be set.
+        localpart = ""
+        if session.chosen_localpart is not None:
+            localpart = session.chosen_localpart
+
         idp_id = session.auth_provider_id
         template_params = {
             "idp": self._sso_handler.get_identity_providers()[idp_id],
             "user_attributes": {
                 "display_name": session.display_name,
                 "emails": session.emails,
+                "localpart": localpart,
             },
         }