diff --git a/CHANGES.md b/CHANGES.md
index 7086b1e9..a5e4667b 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -4,6 +4,7 @@
 
 ### Improvements
 
+-   Write "Unknown cartesian CRS" when saving gdf without a CRS to GPKG (#368).
 -   `read_arrow` and `open_arrow` now provide
     [GeoArrow-compliant extension metadata](https://geoarrow.org/extension-types.html),
     including the CRS, when using GDAL 3.8 or higher (#366).
diff --git a/pyogrio/geopandas.py b/pyogrio/geopandas.py
index 1cbcb113..41b498d2 100644
--- a/pyogrio/geopandas.py
+++ b/pyogrio/geopandas.py
@@ -537,14 +537,19 @@ def write_dataframe(
                 geometry_type = f"{geometry_type} Z"
 
     crs = None
-    if geometry_column is not None and geometry.crs:
-        # TODO: this may need to be WKT1, due to issues
-        # if possible use EPSG codes instead
-        epsg = geometry.crs.to_epsg()
-        if epsg:
-            crs = f"EPSG:{epsg}"
-        else:
-            crs = geometry.crs.to_wkt(WktVersion.WKT1_GDAL)
+    if geometry_column is not None:
+        if geometry.crs:
+            # TODO: this may need to be WKT1, due to issues
+            # if possible use EPSG codes instead
+            epsg = geometry.crs.to_epsg()
+            if epsg:
+                crs = f"EPSG:{epsg}"
+            else:
+                crs = geometry.crs.to_wkt(WktVersion.WKT1_GDAL)
+        elif driver == "GPKG":
+            # In GPKG, None crs must be replaced by "Undefined Cartesian SRS", otherwise
+            # the default "Undefined geographic SRS" will be used.
+            crs = 'LOCAL_CS["Undefined Cartesian SRS"]'
 
     # If there is geometry data, prepare it to be written
     if geometry_column is not None:
diff --git a/pyogrio/tests/test_geopandas_io.py b/pyogrio/tests/test_geopandas_io.py
index 412b95b6..a8f72d31 100644
--- a/pyogrio/tests/test_geopandas_io.py
+++ b/pyogrio/tests/test_geopandas_io.py
@@ -1,6 +1,8 @@
 import contextlib
 from datetime import datetime
 import os
+import sqlite3
+
 import numpy as np
 import pytest
 
@@ -993,6 +995,39 @@ def test_write_dataframe_append(tmp_path, naturalearth_lowres, ext):
     assert len(read_dataframe(output_path)) == 354
 
 
+@pytest.mark.parametrize("ext", [".gpkg", ".shp"])
+def test_write_dataframe_crs_None(tmp_path, ext):
+    input_gdf = gp.GeoDataFrame(geometry=[Point(0, 1)], crs=None)
+    output_path = tmp_path / f"test{ext}"
+
+    write_dataframe(input_gdf, output_path)
+
+    assert os.path.exists(output_path)
+    result_gdf = read_dataframe(output_path)
+
+    if ext == ".gpkg":
+        # In GPKG, cartesian data without specified crs needs to get srs_id -1 according
+        # to the specs: https://www.geopackage.org/spec/#r11
+        # Verify the name of the projection in de GeoDataFrame read.
+        assert result_gdf.crs == 'LOCAL_CS["Undefined Cartesian SRS"]'
+
+        # Verify that srs_id == -1 in the output GPKG file
+        con = sqlite3.connect(output_path)
+        try:
+            result = con.execute(
+                "SELECT srs_id FROM gpkg_geometry_columns WHERE table_name = 'test'"
+            )
+            result_srs_id = result.fetchone()
+        finally:
+            con.close()
+
+        assert result_srs_id is not None
+        assert result_srs_id[0] == -1
+
+    else:
+        assert result_gdf.crs is None
+
+
 @pytest.mark.parametrize("spatial_index", [False, True])
 def test_write_dataframe_gdal_options(tmp_path, naturalearth_lowres, spatial_index):
     df = read_dataframe(naturalearth_lowres)