diff --git a/vector/v.proj/main.c b/vector/v.proj/main.c
index cce7f5d59bf..1a4da0f2d6a 100644
--- a/vector/v.proj/main.c
+++ b/vector/v.proj/main.c
@@ -289,8 +289,13 @@ int main(int argc, char *argv[])
             int first = 1, counter = 0;
             struct Cell_head inwindow;
 
-            G_unset_window();
-            G_get_window(&inwindow);
+            // Initialize from the source projects's default region, not the
+            // current region, because G_get_window() honors WIND_OVERRIDE /
+            // GRASS_REGION, but those refer to a region in the target's mapset,
+            // not in the source project we just switched into. Every spatial
+            // field below is overwritten from the input vector's extent anyway,
+            // so the default window is a sufficient and side-effect-free seed.
+            G_get_default_window(&inwindow);
 
             /* Cycle through all lines */
             Vect_rewind(&Map);
diff --git a/vector/v.proj/tests/v_proj_wind_override_test.py b/vector/v.proj/tests/v_proj_wind_override_test.py
new file mode 100644
index 00000000000..d4fc21011ec
--- /dev/null
+++ b/vector/v.proj/tests/v_proj_wind_override_test.py
@@ -0,0 +1,60 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+"""Regression test for v.proj run with a region override set."""
+
+import os
+from io import StringIO
+
+import pytest
+
+import grass.script as gs
+from grass.tools import Tools
+
+
+@pytest.fixture
+def source_and_target(tmp_path):
+    """Two projects with different CRS.
+
+    The source (lon/lat) holds a vector map in PERMANENT; the target
+    (NC State Plane, meters) is where v.proj runs.
+    """
+    source = tmp_path / "source"
+    target = tmp_path / "target"
+    gs.create_project(source, epsg="4326")
+    gs.create_project(target, epsg="3358")
+    with gs.setup.init(source, env=os.environ.copy()) as session:
+        Tools(session=session).v_in_ascii(
+            input=StringIO("-78.6|35.7\n-78.5|35.8\n"),
+            output="points",
+            separator="|",
+            format="point",
+        )
+    return source, target
+
+
+def test_vproj_succeeds_with_wind_override(source_and_target):
+    """v.proj imports a map while WIND_OVERRIDE names a region only in the
+    target mapset."""
+    source, target = source_and_target
+    with gs.setup.init(target, env=os.environ.copy()) as session:
+        # A named region that exists only in the target's current mapset.
+        gs.run_command(
+            "g.region",
+            n=300000,
+            s=200000,
+            e=700000,
+            w=600000,
+            res=10,
+            save="local_region",
+            env=session.env,
+        )
+        # Set the override after init so it is not stripped by runtime setup;
+        # this mirrors a caller running under a temporary named region.
+        session.env["WIND_OVERRIDE"] = "local_region"
+        Tools(session=session).v_proj(
+            project=source.name,
+            mapset="PERMANENT",
+            input="points",
+            output="points",
+        )
+        assert gs.find_file("points", element="vector", env=session.env)["name"]