diff --git a/synapse/api/auth/oauth_delegated.py b/synapse/api/auth/oauth_delegated.py
new file mode 100644
index 0000000000..b3b5c29a94
--- /dev/null
+++ b/synapse/api/auth/oauth_delegated.py
@@ -0,0 +1,227 @@
+# Copyright 2023 The Matrix.org Foundation.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# 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 logging
+from typing import TYPE_CHECKING, Any, Dict, List, Optional
+from urllib.parse import urlencode
+
+from authlib.oauth2 import ClientAuth
+from authlib.oauth2.auth import encode_client_secret_basic, encode_client_secret_post
+from authlib.oauth2.rfc7523 import ClientSecretJWT, PrivateKeyJWT, private_key_jwt_sign
+from authlib.oauth2.rfc7662 import IntrospectionToken
+from authlib.oidc.discovery import OpenIDProviderMetadata, get_well_known_url
+
+from twisted.web.client import readBody
+from twisted.web.http_headers import Headers
+
+from synapse.api.auth.base import BaseAuth
+from synapse.api.errors import AuthError, StoreError
+from synapse.http.site import SynapseRequest
+from synapse.logging.context import make_deferred_yieldable
+from synapse.types import Requester, UserID, create_requester
+from synapse.util import json_decoder
+from synapse.util.caches.cached_call import RetryOnExceptionCachedCall
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+def scope_to_list(scope: str) -> List[str]:
+    """Convert a scope string to a list of scope tokens"""
+    return scope.strip().split(" ")
+
+
+class PrivateKeyJWTWithKid(PrivateKeyJWT):
+    """An implementation of the private_key_jwt client auth method that includes a kid header.
+
+    This is needed because some providers (Keycloak) require the kid header to figure
+    out which key to use to verify the signature.
+    """
+
+    def sign(self, auth: Any, token_endpoint: str) -> bytes:
+        return private_key_jwt_sign(
+            auth.client_secret,
+            client_id=auth.client_id,
+            token_endpoint=token_endpoint,
+            claims=self.claims,
+            header={"kid": auth.client_secret["kid"]},
+        )
+
+
+class OAuthDelegatedAuth(BaseAuth):
+    AUTH_METHODS = {
+        "client_secret_post": encode_client_secret_post,
+        "client_secret_basic": encode_client_secret_basic,
+        "client_secret_jwt": ClientSecretJWT(),
+        "private_key_jwt": PrivateKeyJWTWithKid(),
+    }
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__(hs)
+
+        self._config = hs.config.auth
+        assert self._config.oauth_delegation_enabled, "OAuth delegation is not enabled"
+        assert self._config.oauth_delegation_issuer, "No issuer provided"
+        assert self._config.oauth_delegation_client_id, "No client_id provided"
+        assert self._config.oauth_delegation_client_secret, "No client_secret provided"
+        assert (
+            self._config.oauth_delegation_client_auth_method
+            in OAuthDelegatedAuth.AUTH_METHODS
+        ), "Invalid client_auth_method"
+
+        self._http_client = hs.get_proxied_http_client()
+        self._hostname = hs.hostname
+
+        self._issuer_metadata = RetryOnExceptionCachedCall(self._load_metadata)
+        secret = self._config.oauth_delegation_client_secret
+        self._client_auth = ClientAuth(
+            self._config.oauth_delegation_client_id,
+            secret,
+            OAuthDelegatedAuth.AUTH_METHODS[
+                self._config.oauth_delegation_client_auth_method
+            ],
+        )
+
+    async def _load_metadata(self) -> OpenIDProviderMetadata:
+        if self._config.oauth_delegation_issuer_metadata is not None:
+            return OpenIDProviderMetadata(
+                **self._config.oauth_delegation_issuer_metadata
+            )
+        url = get_well_known_url(self._config.oauth_delegation_issuer, external=True)
+        response = await self._http_client.get_json(url)
+        metadata = OpenIDProviderMetadata(**response)
+        # metadata.validate_introspection_endpoint()
+        return metadata
+
+    async def _introspect_token(self, token: str) -> IntrospectionToken:
+        metadata = await self._issuer_metadata.get()
+        introspection_endpoint = metadata.get("introspection_endpoint")
+        raw_headers: Dict[str, str] = {
+            "Content-Type": "application/x-www-form-urlencoded",
+            "User-Agent": str(self._http_client.user_agent, "utf-8"),
+            "Accept": "application/json",
+        }
+
+        args = {"token": token, "token_type_hint": "access_token"}
+        body = urlencode(args, True)
+
+        # Fill the body/headers with credentials
+        uri, raw_headers, body = self._client_auth.prepare(
+            method="POST", uri=introspection_endpoint, headers=raw_headers, body=body
+        )
+        headers = Headers({k: [v] for (k, v) in raw_headers.items()})
+
+        # Do the actual request
+        # We're not using the SimpleHttpClient util methods as we don't want to
+        # check the HTTP status code and we do the body encoding ourself.
+        response = await self._http_client.request(
+            method="POST",
+            uri=uri,
+            data=body.encode("utf-8"),
+            headers=headers,
+        )
+
+        resp_body = await make_deferred_yieldable(readBody(response))
+        # TODO: Let's not worry about 5xx errors & co. for now and just try
+        # decoding that as JSON. We should also do some validation of the
+        # response
+        resp = json_decoder.decode(resp_body.decode("utf-8"))
+        return IntrospectionToken(**resp)
+
+    async def get_user_by_req(
+        self,
+        request: SynapseRequest,
+        allow_guest: bool = False,
+        allow_expired: bool = False,
+    ) -> Requester:
+        access_token = self.get_access_token_from_request(request)
+        return await self.get_user_by_access_token(access_token, allow_expired)
+
+    async def get_user_by_access_token(
+        self,
+        token: str,
+        allow_expired: bool = False,
+    ) -> Requester:
+        introspection_result = await self._introspect_token(token)
+
+        logger.info(f"Introspection result: {introspection_result!r}")
+
+        # TODO: introspection verification should be more extensive, especially:
+        #   - verify the scopes
+        #   - verify the audience
+        if not introspection_result.get("active"):
+            raise AuthError(
+                403,
+                "Invalid access token",
+            )
+
+        # TODO: claim mapping should be configurable
+        username: Optional[str] = introspection_result.get("username")
+        if username is None or not isinstance(username, str):
+            raise AuthError(
+                500,
+                "Invalid username claim in the introspection result",
+            )
+
+        # Let's look at the scope
+        scope: List[str] = scope_to_list(introspection_result.get("scope", ""))
+        device_id = None
+        # Find device_id in scope
+        for tok in scope:
+            if tok.startswith("urn:matrix:org.matrix.msc2967.client:device:"):
+                parts = tok.split(":")
+                if len(parts) == 5:
+                    device_id = parts[4]
+
+        user_id = UserID(username, self._hostname)
+        user_info = await self.store.get_userinfo_by_id(user_id=user_id.to_string())
+
+        # If the user does not exist, we should create it on the fly
+        # TODO: we could use SCIM to provision users ahead of time and listen
+        # for SCIM SET events if those ever become standard:
+        # https://datatracker.ietf.org/doc/html/draft-hunt-scim-notify-00
+        if not user_info:
+            await self.store.register_user(user_id=user_id.to_string())
+            user_info = await self.store.get_userinfo_by_id(user_id=user_id.to_string())
+            if not user_info:
+                raise AuthError(
+                    500,
+                    "Could not create user on the fly",
+                )
+
+        if device_id:
+            # Create the device on the fly if it does not exist
+            try:
+                await self.store.get_device(
+                    user_id=user_id.to_string(), device_id=device_id
+                )
+            except StoreError:
+                await self.store.store_device(
+                    user_id=user_id.to_string(),
+                    device_id=device_id,
+                    initial_device_display_name="OIDC-native client",
+                )
+
+        # TODO: there is a few things missing in the requester here, which still need
+        # to be figured out, like:
+        #   - impersonation, with the `authenticated_entity`, which is used for
+        #     rate-limiting, MAU limits, etc.
+        #   - shadow-banning, with the `shadow_banned` flag
+        #   - a proper solution for appservices, which still needs to be figured out in
+        #     the context of MSC3861
+        return create_requester(
+            user_id=user_id,
+            device_id=device_id,
+        )
diff --git a/synapse/config/auth.py b/synapse/config/auth.py
index 35774962c0..25b5cc60dc 100644
--- a/synapse/config/auth.py
+++ b/synapse/config/auth.py
@@ -14,9 +14,11 @@
 # limitations under the License.
 from typing import Any
 
+from authlib.jose.rfc7517 import JsonWebKey
+
 from synapse.types import JsonDict
 
-from ._base import Config
+from ._base import Config, ConfigError
 
 
 class AuthConfig(Config):
@@ -53,3 +55,29 @@ class AuthConfig(Config):
         self.ui_auth_session_timeout = self.parse_duration(
             ui_auth.get("session_timeout", 0)
         )
+
+        oauth_delegation = config.get("oauth_delegation", {})
+        self.oauth_delegation_enabled = oauth_delegation.get("enabled", False)
+        self.oauth_delegation_issuer = oauth_delegation.get("issuer", "")
+        self.oauth_delegation_issuer_metadata = oauth_delegation.get("issuer_metadata")
+        self.oauth_delegation_account = oauth_delegation.get("account", "")
+        self.oauth_delegation_client_id = oauth_delegation.get("client_id", "")
+        self.oauth_delegation_client_secret = oauth_delegation.get("client_secret", "")
+        self.oauth_delegation_client_auth_method = oauth_delegation.get(
+            "client_auth_method", "client_secret_post"
+        )
+
+        self.password_enabled = password_config.get(
+            "enabled", not self.oauth_delegation_enabled
+        )
+
+        if self.oauth_delegation_client_auth_method == "private_key_jwt":
+            self.oauth_delegation_client_secret = JsonWebKey.import_key(
+                self.oauth_delegation_client_secret
+            )
+
+        # If we are delegating via OAuth then password cannot be supported as well
+        if self.oauth_delegation_enabled and self.password_enabled:
+            raise ConfigError(
+                "Password auth cannot be enabled when OAuth delegation is enabled"
+            )
diff --git a/synapse/server.py b/synapse/server.py
index df88af12a9..1c82500f30 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -428,6 +428,10 @@ class HomeServer(metaclass=abc.ABCMeta):
 
     @cache_in_self
     def get_auth(self) -> Auth:
+        if self.config.auth.oauth_delegation_enabled:
+            from synapse.api.auth.oauth_delegated import OAuthDelegatedAuth
+
+            return OAuthDelegatedAuth(self)
         return InternalAuth(self)
 
     @cache_in_self