diff --git a/changelog.d/13239.removal b/changelog.d/13239.removal
new file mode 100644
index 000000000000..8f6045176d44
--- /dev/null
+++ b/changelog.d/13239.removal
@@ -0,0 +1 @@
+Drop support for calling `/_matrix/client/v3/account/3pid/bind` without an `id_access_token`, which was not permitted by the spec. Contributed by @Vetchu.
\ No newline at end of file
diff --git a/synapse/handlers/identity.py b/synapse/handlers/identity.py
index 9bca2bc4b24e..25ce58154c64 100644
--- a/synapse/handlers/identity.py
+++ b/synapse/handlers/identity.py
@@ -163,8 +163,7 @@ async def bind_threepid(
         sid: str,
         mxid: str,
         id_server: str,
-        id_access_token: Optional[str] = None,
-        use_v2: bool = True,
+        id_access_token: str,
     ) -> JsonDict:
         """Bind a 3PID to an identity server
 
@@ -174,8 +173,7 @@ async def bind_threepid(
             mxid: The MXID to bind the 3PID to
             id_server: The domain of the identity server to query
             id_access_token: The access token to authenticate to the identity
-                server with, if necessary. Required if use_v2 is true
-            use_v2: Whether to use v2 Identity Service API endpoints. Defaults to True
+                server with
 
         Raises:
             SynapseError: On any of the following conditions
@@ -187,24 +185,15 @@ async def bind_threepid(
         """
         logger.debug("Proxying threepid bind request for %s to %s", mxid, id_server)
 
-        # If an id_access_token is not supplied, force usage of v1
-        if id_access_token is None:
-            use_v2 = False
-
         if not valid_id_server_location(id_server):
             raise SynapseError(
                 400,
                 "id_server must be a valid hostname with optional port and path components",
             )
 
-        # Decide which API endpoint URLs to use
-        headers = {}
         bind_data = {"sid": sid, "client_secret": client_secret, "mxid": mxid}
-        if use_v2:
-            bind_url = "https://%s/_matrix/identity/v2/3pid/bind" % (id_server,)
-            headers["Authorization"] = create_id_access_token_header(id_access_token)  # type: ignore
-        else:
-            bind_url = "https://%s/_matrix/identity/api/v1/3pid/bind" % (id_server,)
+        bind_url = "https://%s/_matrix/identity/v2/3pid/bind" % (id_server,)
+        headers = {"Authorization": create_id_access_token_header(id_access_token)}
 
         try:
             # Use the blacklisting http client as this call is only to identity servers
@@ -223,21 +212,14 @@ async def bind_threepid(
 
             return data
         except HttpResponseException as e:
-            if e.code != 404 or not use_v2:
-                logger.error("3PID bind failed with Matrix error: %r", e)
-                raise e.to_synapse_error()
+            logger.error("3PID bind failed with Matrix error: %r", e)
+            raise e.to_synapse_error()
         except RequestTimedOutError:
             raise SynapseError(500, "Timed out contacting identity server")
         except CodeMessageException as e:
             data = json_decoder.decode(e.msg)  # XXX WAT?
             return data
 
-        logger.info("Got 404 when POSTing JSON %s, falling back to v1 URL", bind_url)
-        res = await self.bind_threepid(
-            client_secret, sid, mxid, id_server, id_access_token, use_v2=False
-        )
-        return res
-
     async def try_unbind_threepid(self, mxid: str, threepid: dict) -> bool:
         """Attempt to remove a 3PID from an identity server, or if one is not provided, all
         identity servers we're aware the binding is present on
diff --git a/synapse/rest/client/account.py b/synapse/rest/client/account.py
index bdc4a9c0683d..f179fb2606cd 100644
--- a/synapse/rest/client/account.py
+++ b/synapse/rest/client/account.py
@@ -743,10 +743,12 @@ def __init__(self, hs: "HomeServer"):
     async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
         body = parse_json_object_from_request(request)
 
-        assert_params_in_dict(body, ["id_server", "sid", "client_secret"])
+        assert_params_in_dict(
+            body, ["id_server", "sid", "id_access_token", "client_secret"]
+        )
         id_server = body["id_server"]
         sid = body["sid"]
-        id_access_token = body.get("id_access_token")  # optional
+        id_access_token = body["id_access_token"]
         client_secret = body["client_secret"]
         assert_valid_client_secret(client_secret)