diff --git a/airflow/api_connexion/schemas/pool_schema.py b/airflow/api_connexion/schemas/pool_schema.py
index fd42d71e89578..59b58b4adcc36 100644
--- a/airflow/api_connexion/schemas/pool_schema.py
+++ b/airflow/api_connexion/schemas/pool_schema.py
@@ -70,9 +70,18 @@ def get_deferred_slots(obj: Pool) -> int:
         return obj.deferred_slots()
 
     @staticmethod
-    def get_open_slots(obj: Pool) -> float:
-        """Return the open slots of the pool."""
-        return obj.open_slots()
+    def get_open_slots(obj: Pool) -> int:
+        """
+        Return the open slots of the pool.
+
+        Unlimited pools (``slots == -1``) use ``float('inf')`` internally; the REST API
+        schema requires an integer, so we serialize that as ``-1`` (same convention as
+        ``slots`` for unlimited).
+        """
+        open_slots = obj.open_slots()
+        if isinstance(open_slots, float) and open_slots == float("inf"):
+            return -1
+        return int(open_slots)
 
 
 class PoolCollection(NamedTuple):
diff --git a/tests/api_connexion/schemas/test_pool_schemas.py b/tests/api_connexion/schemas/test_pool_schemas.py
index 110103073aaa5..b9a7c6fdecbcc 100644
--- a/tests/api_connexion/schemas/test_pool_schemas.py
+++ b/tests/api_connexion/schemas/test_pool_schemas.py
@@ -33,22 +33,34 @@ def setup_method(self) -> None:
     def teardown_method(self) -> None:
         clear_db_pools()
 
+    @pytest.mark.parametrize(
+        ("pool_name", "slots", "expected_open_slots"),
+        [
+            pytest.param("test_pool", 2, 2, id="finite_slots"),
+            pytest.param("unlimited_pool", -1, -1, id="unlimited_slots_serialized_as_minus_one"),
+        ],
+    )
     @provide_session
-    def test_serialize(self, session):
-        pool_model = Pool(pool="test_pool", slots=2, include_deferred=False)
+    def test_serialize(self, pool_name, slots, expected_open_slots, session):
+        """Pools must serialize open_slots as an integer.
+
+        Unlimited pools (``slots == -1``) must not leak ``inf`` into the JSON
+        response since the OpenAPI schema declares ``open_slots`` as an integer.
+        """
+        pool_model = Pool(pool=pool_name, slots=slots, include_deferred=False)
         session.add(pool_model)
         session.commit()
         pool_instance = session.query(Pool).filter(Pool.pool == pool_model.pool).first()
         serialized_pool = pool_schema.dump(pool_instance)
         assert serialized_pool == {
-            "name": "test_pool",
-            "slots": 2,
+            "name": pool_name,
+            "slots": slots,
             "occupied_slots": 0,
             "running_slots": 0,
             "queued_slots": 0,
             "scheduled_slots": 0,
             "deferred_slots": 0,
-            "open_slots": 2,
+            "open_slots": expected_open_slots,
             "description": None,
             "include_deferred": False,
         }