diff --git a/cid/__init__.py b/cid/__init__.py
index 178a710..5702af0 100644
--- a/cid/__init__.py
+++ b/cid/__init__.py
@@ -8,6 +8,7 @@
     CIDJSONEncoder,
     CIDv0,
     CIDv1,
+    Undef,
     extract_encoding,
     from_bytes,
     from_bytes_strict,
diff --git a/cid/cid.py b/cid/cid.py
index 6f8ad8b..49b3f15 100644
--- a/cid/cid.py
+++ b/cid/cid.py
@@ -186,6 +186,39 @@ def prefix(self) -> "Prefix":
         )
 
 
+class _UndefCID:
+    """Sentinel for undefined/nil CID (go-cid ``Undef``)."""
+
+    _instance: "_UndefCID | None" = None
+
+    def __new__(cls) -> "_UndefCID":
+        if cls._instance is None:
+            cls._instance = cast("_UndefCID", super().__new__(cls))
+        return cls._instance
+
+    def defined(self) -> bool:
+        """Return False; an undefined CID is never defined."""
+        return False
+
+    def __bool__(self) -> bool:
+        return False
+
+    def __repr__(self) -> str:
+        return "CID.Undef"
+
+    def __str__(self) -> str:
+        return "CID.Undef"
+
+    def __eq__(self, other: object) -> bool:
+        return isinstance(other, _UndefCID)
+
+    def __hash__(self) -> int:
+        return hash(_UndefCID)
+
+
+Undef = _UndefCID()
+
+
 class CIDv0(BaseCID):
     """CID version 0 object"""
 
diff --git a/docs/usage.rst b/docs/usage.rst
index 41f9a77..3b84969 100644
--- a/docs/usage.rst
+++ b/docs/usage.rst
@@ -352,7 +352,7 @@ Defined Check
 
 .. code-block:: python
 
-    >>> from cid import CIDv0
+    >>> from cid import CIDv0, Undef
     >>> import multihash
     >>> import hashlib
     >>>
@@ -363,6 +363,12 @@ Defined Check
     >>> cid = CIDv0(mhash)
     >>> cid.defined()
     True
+    >>>
+    >>> # Undef is a sentinel for an undefined/nil CID (like go-cid Undef)
+    >>> Undef.defined()
+    False
+    >>> bool(Undef)
+    False
 
 Stream Parsing
 --------------
diff --git a/newsfragments/68.feature.rst b/newsfragments/68.feature.rst
new file mode 100644
index 0000000..6a630f2
--- /dev/null
+++ b/newsfragments/68.feature.rst
@@ -0,0 +1 @@
+Add ``Undef`` sentinel for representing undefined/nil CIDs, matching go-cid ``Undef``.
diff --git a/tests/test_new_features.py b/tests/test_new_features.py
index afb570d..a44a2bd 100644
--- a/tests/test_new_features.py
+++ b/tests/test_new_features.py
@@ -10,6 +10,7 @@
     CIDSet,
     CIDv0,
     CIDv1,
+    Undef,
     V0Builder,
     V1Builder,
     extract_encoding,
@@ -19,6 +20,7 @@
     must_parse,
     parse_ipfs_path,
 )
+from cid.cid import _UndefCID
 
 
 @pytest.fixture
@@ -321,6 +323,37 @@ def test_defined_cidv1(self, cidv1):
         assert cidv1.defined() is True
 
 
+class TestUndef:
+    """Tests for Undef sentinel"""
+
+    def test_undef_defined_is_false(self):
+        assert Undef.defined() is False
+
+    def test_undef_bool_is_false(self):
+        assert bool(Undef) is False
+
+    def test_undef_repr_and_str(self):
+        assert repr(Undef) == "CID.Undef"
+        assert str(Undef) == "CID.Undef"
+
+    def test_undef_is_singleton(self):
+        assert Undef is Undef
+        assert _UndefCID() is Undef
+
+    def test_undef_equality(self, cidv0):
+        assert Undef == Undef
+        assert Undef == _UndefCID()
+        assert Undef != cidv0
+
+    def test_undef_hashable(self):
+        assert Undef in {Undef}
+        mapping = {Undef: "unset"}
+        assert mapping[Undef] == "unset"
+
+    def test_undef_not_cid_instance(self):
+        assert not isinstance(Undef, (CIDv0, CIDv1))
+
+
 class TestFromReader:
     """Tests for from_reader function"""