diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3f60813..82fd098 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -79,5 +79,10 @@ jobs:
       - name: pytest
         run: ci/pytest_unit.sh
 
-      - name: perf
+      - name: test performance
         run: ci/pytest_perf.sh
+
+      - name: test compatibility with Cirq
+        run: |
+          pip install cirq-core
+          pytest test/* -m cirq
diff --git a/pyproject.toml b/pyproject.toml
index 4e24403..5cb2a24 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -11,4 +11,8 @@ requires = ["setuptools", "wheel", "Cython"]
 testpaths = [
     "test",
     "test_perf",
-]
\ No newline at end of file
+]
+markers = [
+    "cirq: mark a test as a compatibility test with Cirq.",
+]
+addopts = '-m "not cirq"'
diff --git a/test/cirq_compatibility.py b/test/cirq_compatibility.py
new file mode 100644
index 0000000..bc8fc6e
--- /dev/null
+++ b/test/cirq_compatibility.py
@@ -0,0 +1,43 @@
+# Copyright 2025 The TUnits Authors
+#
+# 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 pytest
+import tunits as tu
+import cirq  # type: ignore
+
+
+@pytest.mark.cirq
+def test_to_json() -> None:
+    assert (
+        cirq.to_json(tu.ns * 3)
+        == """{
+  "cirq_type": "tunits.Value",
+  "value": 3,
+  "unit": "ns"
+}"""
+    )
+
+    assert (
+        cirq.to_json(tu.GHz * [1, 2, 3, -1])
+        == """{
+  "cirq_type": "tunits.ValueArray",
+  "value": [
+    1.0,
+    2.0,
+    3.0,
+    -1.0
+  ],
+  "unit": "GHz"
+}"""
+    )
diff --git a/tunits/core/__init__.pyi b/tunits/core/__init__.pyi
index 4f00248..a52feb3 100644
--- a/tunits/core/__init__.pyi
+++ b/tunits/core/__init__.pyi
@@ -214,6 +214,11 @@ class WithUnit:
     def __divmod__(self, other: Any) -> tuple[_NUMERICAL_TYPE_OR_ARRAY, 'WithUnit']: ...
     def _value_class(self) -> type['Value']: ...
     def _array_class(self) -> type['ValueArray']: ...
+    def _json_dict_(self) -> dict[str, Any]: ...
+    @classmethod
+    def _json_namespace_(cls) -> str: ...
+    @classmethod
+    def _from_json_dict_(cls: type[T], **kwargs: Any) -> T: ...
 
 class Value(Generic[NumericalT], WithUnit, np.generic, SupportsIndex):
     """A floating-point value with associated units."""
diff --git a/tunits/core/cython/with_unit.pyx b/tunits/core/cython/with_unit.pyx
index c910bd0..4907378 100644
--- a/tunits/core/cython/with_unit.pyx
+++ b/tunits/core/cython/with_unit.pyx
@@ -674,6 +674,17 @@ cdef class WithUnit:
             raise ValueError(f'{self} is not dimensionless')
         return self.value_in_base_units()
 
+    def _json_dict_(self) -> dict[str, Any]:
+        return {"value": self.value, "unit": self.units}
+
+    @classmethod
+    def _json_namespace_(cls) -> str:
+        return "tunits"
+
+    @classmethod
+    def _from_json_dict_(cls, **kwargs):
+        return cls(kwargs["value"], kwargs["unit"])
+
 _try_interpret_as_with_unit = None
 _is_value_consistent_with_default_unit_database = None
 def init_base_unit_functions(