diff --git a/.github/workflows/osgeo4w.yml b/.github/workflows/osgeo4w.yml
index 96b5a6bb9aa..dcf30d6542e 100644
--- a/.github/workflows/osgeo4w.yml
+++ b/.github/workflows/osgeo4w.yml
@@ -98,6 +98,25 @@ jobs:
             sqlite3-devel
             zstd-devel
 
+      - name: "[CI diagnostic, not for merge] Revert gdal/python3-gdal to last known-good 3.13.3-1"
+        # Bisects the 2026-09-25 OSGeo4W regression (gdal 3.13.3-1 -> 3.13.3-2)
+        # by force-installing the previous build over the freshly installed one.
+        shell: pwsh
+        env:
+          O4WROOT: ${{ steps.osgeo4w.outputs.root }}
+        run: |
+          $ErrorActionPreference = 'Stop'
+          # gdal313.dll ships in gdal313-runtime, not in the gdal package itself.
+          $base = 'https://download.osgeo.org/osgeo4w/v2/x86_64/release/gdal'
+          foreach ($pkg in @('gdal313-runtime', 'python3-gdal')) {
+            $file = "$pkg-3.13.3-1.tar.bz2"
+            $url = "$base/$pkg/$file"
+            Write-Output "Downloading $url"
+            Invoke-WebRequest $url -OutFile $file
+            Write-Output "Extracting $file over $env:O4WROOT (overwrites the 3.13.3-2 build)"
+            tar -xjf $file -C $env:O4WROOT
+          }
+
       - name: Set number of cores for compilation
         run: |
           echo "MAKEFLAGS=-j$(nproc)" >> "${GITHUB_ENV}"