diff --git a/src/fromager/sources.py b/src/fromager/sources.py
index 996eb333..bd308adb 100644
--- a/src/fromager/sources.py
+++ b/src/fromager/sources.py
@@ -212,6 +212,15 @@ def default_download_source(
     url = pbi.download_source_url(version=version, default=download_url)
     if url is None:
         raise ValueError(f"Could not determine download URL for {req}")
+    if destination_filename is None:
+        url_filename = resolver.extract_filename_from_url(url)
+        if url_filename.endswith(".zip"):
+            destination_filename = f"{pbi.override_module_name}-{version}.zip"
+        else:
+            destination_filename = f"{pbi.override_module_name}-{version}.tar.gz"
+        logger.debug(
+            "config has no destination_filename, use default %r", destination_filename
+        )
     source_filename = _download_source_check(
         req=req,
         destination_dir=sdists_downloads_dir,
diff --git a/tests/test_sources.py b/tests/test_sources.py
index 19ad7f06..57f18478 100644
--- a/tests/test_sources.py
+++ b/tests/test_sources.py
@@ -166,6 +166,48 @@ def test_patch_sources_apply_only_unversioned(
     apply_patch.assert_called_once_with(req, unversioned_patch_file, source_root_dir)
 
 
+@pytest.mark.parametrize(
+    "pkg,version_str,url,expected_filename",
+    [
+        (
+            "mlserver-sklearn==1.6.0",
+            "1.6.0",
+            "https://github.test/SeldonIO/MLServer/archive/refs/tags/1.6.0.tar.gz",
+            "mlserver_sklearn-1.6.0.tar.gz",
+        ),
+        (
+            "some-pkg==2.0",
+            "2.0",
+            "https://github.test/owner/repo/archive/refs/tags/v2.0.zip",
+            "some_pkg-2.0.zip",
+        ),
+    ],
+)
+@patch("fromager.sources._download_source_check")
+def test_default_download_source_no_destination_filename(
+    download_source_check: Mock,
+    tmp_context: context.WorkContext,
+    pkg: str,
+    version_str: str,
+    url: str,
+    expected_filename: str,
+) -> None:
+    """When no destination_filename is configured, use PEP 625 normalized name."""
+    req = Requirement(pkg)
+    version = Version(version_str)
+
+    sources.default_download_source(
+        tmp_context, req, version, url, tmp_context.sdists_downloads
+    )
+
+    download_source_check.assert_called_with(
+        req=req,
+        destination_dir=tmp_context.sdists_downloads,
+        url=url,
+        destination_filename=expected_filename,
+    )
+
+
 @patch("fromager.sources.vendor_rust.vendor_rust")
 @patch("fromager.sources.pyproject.apply_project_override")
 @patch("fromager.sources.patch_source")