diff --git a/src/power_grid_model_ds/_core/model/grids/serialization/json.py b/src/power_grid_model_ds/_core/model/grids/serialization/json.py
index 635e20e7..9998bf17 100644
--- a/src/power_grid_model_ds/_core/model/grids/serialization/json.py
+++ b/src/power_grid_model_ds/_core/model/grids/serialization/json.py
@@ -7,6 +7,7 @@
 import dataclasses
 import json
 import logging
+import math
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
 
@@ -144,7 +145,7 @@ def _restore_grid_values[G: Grid](grid: G, json_data: dict) -> None:
 
 
 def _serialize_array(array: FancyArray) -> list[dict[str, Any]]:
-    return [{name: record[name].item() for name in array.columns} for record in array]
+    return [{name: _replace_nan_with_none(record[name].item()) for name in array.columns} for record in array]
 
 
 def _deserialize_array(array_data: list[dict[str, Any]], array_class: type[FancyArray]) -> FancyArray:
@@ -182,3 +183,10 @@ def _is_serializable(value: Any, strict: bool, **kwargs) -> bool:
         _logger.warning(msg)
         return False
     return True
+
+
+def _replace_nan_with_none(value: Any) -> Any:
+    """Replace a NaN value with a JSON-compatible null value."""
+    if isinstance(value, float) and math.isnan(value):
+        return None
+    return value
diff --git a/tests/unit/model/grids/serialization/data/legacy_nan.json b/tests/unit/model/grids/serialization/data/legacy_nan.json
new file mode 100644
index 00000000..81f28e15
--- /dev/null
+++ b/tests/unit/model/grids/serialization/data/legacy_nan.json
@@ -0,0 +1,10 @@
+{
+  "data": {
+    "node": [
+      {
+        "id": 1,
+        "u_rated": NaN
+      }
+    ]
+  }
+}
diff --git a/tests/unit/model/grids/serialization/data/legacy_nan.json.license b/tests/unit/model/grids/serialization/data/legacy_nan.json.license
new file mode 100644
index 00000000..76010591
--- /dev/null
+++ b/tests/unit/model/grids/serialization/data/legacy_nan.json.license
@@ -0,0 +1,3 @@
+SPDX-FileCopyrightText: Contributors to the Power Grid Model project <powergridmodel@lfenergy.org>
+
+SPDX-License-Identifier: MPL-2.0
diff --git a/tests/unit/model/grids/serialization/test_json.py b/tests/unit/model/grids/serialization/test_json.py
index 210d9a96..bb66abf1 100644
--- a/tests/unit/model/grids/serialization/test_json.py
+++ b/tests/unit/model/grids/serialization/test_json.py
@@ -155,6 +155,25 @@ def test_pgm_roundtrip(self, request, grid_fixture: str, tmp_path: Path):
 
             assert array_equal_with_nan(original_array, loaded_array), f"Array '{array_name}' does not match"
 
+    def test_nan_serializes_as_json_null(self, basic_grid: Grid, tmp_path: Path):
+        basic_grid.node.u_rated[0] = np.nan
+
+        path = basic_grid.serialize(tmp_path / "grid.json")
+        with path.open(encoding="utf-8") as file:
+            file_data = json.load(file)
+
+        string_data = json.loads(basic_grid.serialize(mode="json_string"))
+
+        assert file_data["data"]["node"][0]["u_rated"] is None
+        assert string_data["data"]["node"][0]["u_rated"] is None
+
+    def test_deserialize_legacy_nan_json(self):
+        path = Path(__file__).parent / "data" / "legacy_nan.json"
+
+        grid = Grid.deserialize(path)
+
+        assert np.isnan(grid.node.u_rated[0])
+
 
 class TestCrossTypeCompatibility:
     """Test cross-type loading and compatibility"""