diff --git a/src/neris_api_client/client.py b/src/neris_api_client/client.py
index 0c4b63d..814db66 100644
--- a/src/neris_api_client/client.py
+++ b/src/neris_api_client/client.py
@@ -182,6 +182,8 @@ def _call(
                 data = model.model_validate_json(data).model_dump(mode="json", by_alias=True)
             if isinstance(data, dict):
                 data = model.model_validate(data).model_dump(mode="json", by_alias=True)
+        elif isinstance(data, str):
+            data = json.loads(data)
 
         res = getattr(self._session, method)(
             f"{self.config.base_url}{path}", json=data, params=params
diff --git a/tests/test_payload_serialization.py b/tests/test_payload_serialization.py
new file mode 100644
index 0000000..acbeaf6
--- /dev/null
+++ b/tests/test_payload_serialization.py
@@ -0,0 +1,40 @@
+from datetime import datetime
+from types import SimpleNamespace
+import unittest
+
+from neris_api_client.client import _NerisApiClient
+
+
+class Response:
+    def raise_for_status(self):
+        pass
+
+    def json(self):
+        return {}
+
+
+class Session:
+    def __init__(self):
+        self.headers = {}
+        self.kwargs = None
+
+    def post(self, *args, **kwargs):
+        self.kwargs = kwargs
+        return Response()
+
+
+class PayloadSerializationTests(unittest.TestCase):
+    def test_json_string_payload_is_sent_as_a_json_object_without_validation(self):
+        client = object.__new__(_NerisApiClient)
+        client.config = SimpleNamespace(base_url="https://api.example.test", debug=False, validate=False)
+        client.tokens = SimpleNamespace(access_token="token", expires_at=datetime.max)
+        client._session = Session()
+        client._update_auth = lambda: None
+
+        client._call("post", "/entity", data='{"name": "Example Fire Department"}')
+
+        self.assertEqual(client._session.kwargs["json"], {"name": "Example Fire Department"})
+
+
+if __name__ == "__main__":
+    unittest.main()