@@ -31,6 +31,7 @@ def __init__(
         claims=None,
         token_placement="header",
         scope=None,
+        client_id=None,
         **kwargs,
     ):
         client_kwargs = extract_client_kwargs(kwargs)
@@ -47,6 +48,7 @@ def __init__(
             claims=claims,
             token_placement=token_placement,
             scope=scope,
+            client_id=client_id,
             **kwargs,
         )
 
@@ -88,6 +90,7 @@ def __init__(
         claims=None,
         token_placement="header",
         scope=None,
+        client_id=None,
         **kwargs,
     ):
         client_kwargs = extract_client_kwargs(kwargs)
@@ -104,6 +107,7 @@ def __init__(
             claims=claims,
             token_placement=token_placement,
             scope=scope,
+            client_id=client_id,
             **kwargs,
         )
 
 
@@ -39,6 +39,7 @@ def __init__(
         claims=None,
         token_placement="header",
         scope=None,
+        client_id=None,
         default_timeout=None,
         leeway=60,
         **kwargs,
@@ -57,6 +58,7 @@ def __init__(
             claims=claims,
             token_placement=token_placement,
             scope=scope,
+            client_id=client_id,
             leeway=leeway,
             **kwargs,
         )
 
@@ -25,6 +25,7 @@ def __init__(
         claims=None,
         token_placement="header",
         scope=None,
+        client_id=None,
         leeway=60,
         **kwargs,
     ):
@@ -46,6 +47,7 @@ def __init__(
         self.audience = audience
         self.claims = claims
         self.scope = scope
+        self.client_id = client_id
         if self.token_auth_class is not None:
             self.token_auth = self.token_auth_class(None, token_placement, self)
         self._kwargs = kwargs
@@ -79,6 +81,8 @@ def refresh_token(self):
         }
         if self.scope:
             data["scope"] = self.scope
+        if self.client_id:
+            data["client_id"] = self.client_id
 
         return self._refresh_token(data)
 
 
@@ -90,6 +90,9 @@ Version 1.7.0
 - RFC7523 accepts the issuer URL as a valid audience. :issue:`730`
 - Fix ``InvalidTokenError`` extra attributes being wrapped instead of passed as
   individual key=value pairs in the ``WWW-Authenticate`` header. :pr:`872`
+- Add optional ``client_id`` parameter to ``AssertionClient`` (and its
+  ``AssertionSession`` / httpx wrappers) so it can be sent in the token
+  endpoint request body per :rfc:`7521` §4.1 and :rfc:`6749` §3.2.1. :pr:`476`
 
 Upgrade Guide: :ref:`joserfc_upgrade`.
 
 
@@ -52,6 +52,28 @@ def verifier(request):
         client.get("https://provider.test")
 
 
+def test_client_id():
+    """client_id is included in the token request body when provided."""
+
+    def verifier(request):
+        if str(request.url) == "https://provider.test/token":
+            content = request.form
+            assert "assertion" in content
+            assert content["client_id"] == "my-client"
+
+    with AssertionClient(
+        "https://provider.test/token",
+        issuer="foo",
+        subject="foo",
+        audience="foo",
+        alg="HS256",
+        key="secret",
+        client_id="my-client",
+        transport=WSGITransport(MockDispatch(default_token, assert_func=verifier)),
+    ) as client:
+        client.get("https://provider.test")
+
+
 def test_without_alg():
     with AssertionClient(
         "https://provider.test/token",
 
@@ -54,6 +54,29 @@ async def verifier(request):
         await client.get("https://provider.test")
 
 
+@pytest.mark.asyncio
+async def test_client_id():
+    """client_id is included in the token request body when provided."""
+
+    async def verifier(request):
+        if str(request.url) == "https://provider.test/token":
+            content = await request.body()
+            assert b"assertion=" in content
+            assert b"client_id=my-client" in content
+
+    async with AsyncAssertionClient(
+        "https://provider.test/token",
+        issuer="foo",
+        subject="foo",
+        audience="foo",
+        alg="HS256",
+        key="secret",
+        client_id="my-client",
+        transport=ASGITransport(AsyncMockDispatch(default_token, assert_func=verifier)),
+    ) as client:
+        await client.get("https://provider.test")
+
+
 @pytest.mark.asyncio
 async def test_without_alg():
     async with AsyncAssertionClient(