diff --git a/mesonbuild/msubprojects.py b/mesonbuild/msubprojects.py
index f4b4405634b2..3e61b7c3439f 100755
--- a/mesonbuild/msubprojects.py
+++ b/mesonbuild/msubprojects.py
@@ -720,7 +720,14 @@ def add_arguments(parser: argparse.ArgumentParser) -> None:
     p.set_defaults(subprojects_func=Runner.packagefiles)
 
 def run(options: 'Arguments') -> int:
-    source_dir = os.path.relpath(os.path.realpath(options.sourcedir))
+    real_source_dir = os.path.realpath(options.sourcedir)
+    try:
+        source_dir = os.path.relpath(real_source_dir)
+    except ValueError:
+        # On Windows, os.path.relpath raises ValueError when the source
+        # directory is on a different mount/UNC share than the current
+        # working directory. Fall back to the absolute path in that case.
+        source_dir = real_source_dir
     if not os.path.isfile(os.path.join(source_dir, 'meson.build')):
         mlog.error('Directory', mlog.bold(source_dir), 'does not seem to be a Meson source directory.')
         return 1
diff --git a/unittests/machinefiletests.py b/unittests/machinefiletests.py
index b20b8172f9e4..d0e5868038a1 100644
--- a/unittests/machinefiletests.py
+++ b/unittests/machinefiletests.py
@@ -93,6 +93,30 @@ def test_home_variable(self):
         finally:
             os.unlink(fname)
 
+class SubprojectsUpdateTests(TestCase):
+
+    def test_subprojects_update_relpath_cross_mount(self):
+        """msubprojects.run() must not crash with an unhandled ValueError
+        when os.path.relpath() cannot compute a relative path (as happens
+        on Windows when the source directory is on a different mount/UNC
+        share than the current working directory)."""
+        import mesonbuild.msubprojects as msubprojects
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            options = mock.Mock()
+            options.sourcedir = tmpdir
+            options.allow_insecure = False
+
+            def raising_relpath(*args, **kwargs):
+                raise ValueError("path is on mount 'Z:', start on mount 'C:'")
+
+            with mock.patch('os.path.relpath', side_effect=raising_relpath):
+                # No meson.build in tmpdir, so run() should reach the
+                # "does not seem to be a Meson source directory" error
+                # path and return 1, rather than raising ValueError.
+                result = msubprojects.run(options)
+            self.assertEqual(result, 1)
+
 class NativeFileTests(BasePlatformTests):
 
     def setUp(self):