diff --git a/lib/vector/Vlib/open.c b/lib/vector/Vlib/open.c
index d6d78fc25d8..ecb7e9f28c9 100644
--- a/lib/vector/Vlib/open.c
+++ b/lib/vector/Vlib/open.c
@@ -1291,6 +1291,12 @@ int Vect_open_sidx(struct Map_info *Map, int mode)
                 fclose(Plus->spidx_fp.file);
                 return -1;
             }
+            if (!Plus->Spidx_file) {
+                /* the index is fully loaded in memory and the file is
+                 * rewritten by Vect_save_sidx() */
+                fclose(Plus->spidx_fp.file);
+                Plus->spidx_fp.file = NULL;
+            }
         }
     }
 
diff --git a/python/grass/pygrass/vector/table.py b/python/grass/pygrass/vector/table.py
index ea54af90841..7c92256642f 100644
--- a/python/grass/pygrass/vector/table.py
+++ b/python/grass/pygrass/vector/table.py
@@ -48,7 +48,7 @@ def get_path(path, vect_name=None):
     ...     "sqlite",
     ...     "sqlite.db",
     ... )
-    >>> new_path.replace("//", "/") == new_path2.replace("//", "/")
+    >>> os.path.normpath(new_path) == os.path.normpath(new_path2)
     True
     >>> path = "$GISDBASE/$LOCATION_NAME/$MAPSET/vector/$MAP/sqlite.db"
     >>> new_path = get_path(path, "test")
@@ -60,7 +60,7 @@ def get_path(path, vect_name=None):
     ...     "test",
     ...     "sqlite.db",
     ... )
-    >>> new_path.replace("//", "/") == new_path2.replace("//", "/")
+    >>> os.path.normpath(new_path) == os.path.normpath(new_path2)
     True
 
     """
diff --git a/python/grass/pygrass/vector/tests/grass_pygrass_vector_sidx_checks.py b/python/grass/pygrass/vector/tests/grass_pygrass_vector_sidx_checks.py
new file mode 100644
index 00000000000..b50e033d3db
--- /dev/null
+++ b/python/grass/pygrass/vector/tests/grass_pygrass_vector_sidx_checks.py
@@ -0,0 +1,49 @@
+# Copyright (C) 2026 by the GRASS Development Team
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+"""Open a vector map in update mode, close it, and check for a leaked sidx handle
+
+Run in a GRASS session by ``grass_pygrass_vector_sidx_test.py`` in a
+subprocess, because the vector library state is per process.
+"""
+
+import os
+import sys
+from pathlib import Path
+
+from grass.pygrass.vector import VectorTopo
+
+
+def sidx_is_open(path):
+    """Return True if this process still has the file open"""
+    if sys.platform.startswith("linux"):
+        # A handle to a replaced file is reported with a " (deleted)" suffix.
+        return any(
+            os.path.realpath(fd).startswith(path)
+            for fd in Path("/proc/self/fd").iterdir()
+        )
+    # Windows refuses to delete an open file.
+    try:
+        os.remove(path)
+    except PermissionError:
+        return True
+    return False
+
+
+def main():
+    """Create a map, reopen it in update mode, and check the sidx file"""
+    vect = VectorTopo("sidx_map")
+    vect.open("w")
+    vect.close()
+    vect.open("rw")
+    vect.close()
+    mapset_path = os.environ["MAPSET_PATH"]
+    path = os.path.realpath(os.path.join(mapset_path, "vector", "sidx_map", "sidx"))
+    if sidx_is_open(path):
+        print(f"The file {path} was left open after closing the map")
+        return 1
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/python/grass/pygrass/vector/tests/grass_pygrass_vector_sidx_test.py b/python/grass/pygrass/vector/tests/grass_pygrass_vector_sidx_test.py
new file mode 100644
index 00000000000..ec3c9d98849
--- /dev/null
+++ b/python/grass/pygrass/vector/tests/grass_pygrass_vector_sidx_test.py
@@ -0,0 +1,47 @@
+# Copyright (C) 2026 by the GRASS Development Team
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+"""Test that closing a vector map opened in update mode releases the sidx file
+
+An open file handle left behind makes the removal of the map fail on Windows
+and leaks a file descriptor on all platforms.
+"""
+
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+import grass.script as gs
+
+CHECKS = Path(__file__).with_name("grass_pygrass_vector_sidx_checks.py")
+
+
+@pytest.fixture(scope="module")
+def session(tmp_path_factory):
+    """Active session in an XY project (scope: module)"""
+    project = tmp_path_factory.mktemp("sidx") / "project"
+    gs.create_project(project)
+    with gs.setup.init(project, env=os.environ.copy()) as session:
+        yield session
+
+
+def test_update_mode_releases_sidx(session):
+    """The sidx file is closed after a map opened with "rw" is closed"""
+    env = session.env.copy()
+    env["MAPSET_PATH"] = str(
+        Path(gs.gisenv(env=session.env)["GISDBASE"])
+        / gs.gisenv(env=session.env)["LOCATION_NAME"]
+        / gs.gisenv(env=session.env)["MAPSET"]
+    )
+    result = subprocess.run(
+        [sys.executable, str(CHECKS)],
+        env=env,
+        capture_output=True,
+        text=True,
+        check=False,
+        timeout=120,
+    )
+    assert result.returncode == 0, f"{result.stdout}\n{result.stderr}"