diff --git a/changelog.d/8180.feature b/changelog.d/8180.feature
new file mode 100644
index 000000000000..4ee5b6a56e37
--- /dev/null
+++ b/changelog.d/8180.feature
@@ -0,0 +1 @@
+Add a configuration option that allows existing users to log in with OpenID Connect. Contributed by @BBBSnowball and @OmmyZhang.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 2a5b2e093537..6afe71b24f78 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -1711,6 +1711,10 @@ oidc_config:
   #
   #skip_verification: true
 
+  # Uncomment to add oidc token to the account instead of failing when user already exists. Defaults to false.
+  #
+  #allow_existing_users: true
+
   # 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/config/oidc_config.py b/synapse/config/oidc_config.py
index e0939bce84a1..c9561771c892 100644
--- a/synapse/config/oidc_config.py
+++ b/synapse/config/oidc_config.py
@@ -56,6 +56,7 @@ def read_config(self, config, **kwargs):
         self.oidc_userinfo_endpoint = oidc_config.get("userinfo_endpoint")
         self.oidc_jwks_uri = oidc_config.get("jwks_uri")
         self.oidc_skip_verification = oidc_config.get("skip_verification", False)
+        self.oidc_allow_existing_users = oidc_config.get("allow_existing_users", False)
 
         ump_config = oidc_config.get("user_mapping_provider", {})
         ump_config.setdefault("module", DEFAULT_USER_MAPPING_PROVIDER)
@@ -158,6 +159,10 @@ def generate_config_section(self, config_dir_path, server_name, **kwargs):
           #
           #skip_verification: true
 
+          # Uncomment to add oidc token to the account instead of failing when user already exists. Defaults to false.
+          #
+          #allow_existing_users: true
+
           # 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 1b06f3173fa0..482997eaa2b3 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -114,6 +114,7 @@ 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._allow_existing_users = hs.config.oidc_allow_existing_users  # type: bool
 
         self._http_client = hs.get_proxied_http_client()
         self._auth_handler = hs.get_auth_handler()
@@ -849,7 +850,8 @@ async def _map_userinfo_to_user(
         If we don't find the user that way, we should register the user,
         mapping the localpart and the display name from the UserInfo.
 
-        If a user already exists with the mxid we've mapped, raise an exception.
+        If a user already exists with the mxid we've mapped and allow_existing_users
+        is disabled , raise an exception.
 
         Args:
             userinfo: an object representing the user
@@ -906,20 +908,25 @@ async def _map_userinfo_to_user(
         localpart = map_username_to_mxid_localpart(attributes["localpart"])
 
         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())
-            )
-
-        # 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"],
-            user_agent_ips=(user_agent, ip_address),
+        matches = await self._datastore.get_users_by_id_case_insensitive(
+            user_id.to_string()
         )
-
+        if matches:
+            if self._allow_existing_users:
+                registered_user_id = next(iter(matches))
+            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"],
+                user_agent_ips=(user_agent, ip_address),
+            )
         await self._datastore.record_user_external_id(
             self._auth_provider_id, remote_user_id, registered_user_id,
         )