diff --git a/synapse/config/oidc_config.py b/synapse/config/oidc_config.py
index 586038078f86..f1a3f9d1df5a 100644
--- a/synapse/config/oidc_config.py
+++ b/synapse/config/oidc_config.py
@@ -51,12 +51,14 @@ def read_config(self, config, **kwargs):
             "client_auth_method", "client_secret_basic"
         )
         self.oidc_scopes = oidc_config.get("scopes", ["openid"])
+        self.oidc_uses_userinfo = oidc_config.get("uses_userinfo", False)
         self.oidc_authorization_endpoint = oidc_config.get("authorization_endpoint")
         self.oidc_token_endpoint = oidc_config.get("token_endpoint")
         self.oidc_userinfo_endpoint = oidc_config.get("userinfo_endpoint")
         self.oidc_jwks_uri = oidc_config.get("jwks_uri")
         self.oidc_subject_claim = oidc_config.get("subject_claim", "sub")
         self.oidc_skip_verification = oidc_config.get("skip_verification", False)
+        self.oidc_merge_with_existing_users = oidc_config.get("merge_with_existing_users", False)
 
         ump_config = oidc_config.get("user_mapping_provider", {})
         ump_config.setdefault("module", DEFAULT_USER_MAPPING_PROVIDER)
@@ -118,6 +120,11 @@ def generate_config_section(self, config_dir_path, server_name, **kwargs):
             #
             #scopes: ["openid"]
 
+            # always use userinfo endpoint. This is required for providers that don't include user
+            # information in the token response, e.g. Gitlab.
+            #
+            #uses_userinfo: false
+
             # the oauth2 authorization endpoint. Required if provider discovery is disabled.
             #
             #authorization_endpoint: "https://accounts.example.com/oauth2/auth"
@@ -140,6 +147,10 @@ def generate_config_section(self, config_dir_path, server_name, **kwargs):
             #
             #skip_verification: false
 
+            # if user already exists, add oidc token to that account instead of failing. Defaults to false.
+            #
+            #merge_with_existing_users: false
+
 
             # An external module can be provided here as a custom solution to mapping
             # attributes returned from a OIDC provider onto a matrix user.
diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
index 4ba8c7fda502..7d475ded2ce0 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -93,6 +93,7 @@ class OidcHandler:
     def __init__(self, hs: HomeServer):
         self._callback_url = hs.config.oidc_callback_url  # type: str
         self._scopes = hs.config.oidc_scopes  # type: List[str]
+        self._uses_userinfo_config = hs.config.oidc_uses_userinfo  # type: bool
         self._client_auth = ClientAuth(
             hs.config.oidc_client_id,
             hs.config.oidc_client_secret,
@@ -112,6 +113,7 @@ def __init__(self, hs: HomeServer):
             hs.config.oidc_user_mapping_provider_config
         )  # type: OidcMappingProvider
         self._skip_verification = hs.config.oidc_skip_verification  # type: bool
+        self._merge_with_existing_users = hs.config.oidc_merge_with_existing_users  # type: bool
 
         self._http_client = hs.get_proxied_http_client()
         self._auth_handler = hs.get_auth_handler()
@@ -224,8 +226,7 @@ def _uses_userinfo(self) -> bool:
         ``access_token`` with the ``userinfo_endpoint``.
         """
 
-        # Maybe that should be user-configurable and not inferred?
-        return "openid" not in self._scopes
+        return self._uses_userinfo_config or "openid" not in self._scopes
 
     async def load_metadata(self) -> OpenIDProviderMetadata:
         """Load and validate the provider metadata.
@@ -884,17 +885,20 @@ async def _map_userinfo_to_user(self, userinfo: UserInfo, token: Token) -> str:
 
         user_id = UserID(localpart, self._hostname)
         if await self._datastore.get_users_by_id_case_insensitive(user_id.to_string()):
-            # This mxid is taken
-            raise MappingException(
-                "mxid '{}' is already taken".format(user_id.to_string())
+            if self._merge_with_existing_users:
+                registered_user_id = user_id.to_string()
+            else:
+                # This mxid is taken
+                raise MappingException(
+                    "mxid '{}' is already taken".format(user_id.to_string())
+                )
+        else:
+            # It's the first time this user is logging in and the mapped mxid was
+            # not taken, register the user
+            registered_user_id = await self._registration_handler.register_user(
+                localpart=localpart, default_display_name=attributes["display_name"],
             )
 
-        # It's the first time this user is logging in and the mapped mxid was
-        # not taken, register the user
-        registered_user_id = await self._registration_handler.register_user(
-            localpart=localpart, default_display_name=attributes["display_name"],
-        )
-
         await self._datastore.record_user_external_id(
             self._auth_provider_id, remote_user_id, registered_user_id,
         )