diff --git a/go/cmd/generatesitemap/generatesitemap.go b/go/cmd/generatesitemap/generatesitemap.go
index 49136fcb212..430f28e53f2 100644
--- a/go/cmd/generatesitemap/generatesitemap.go
+++ b/go/cmd/generatesitemap/generatesitemap.go
@@ -230,9 +230,9 @@ func generateSitemaps(ctx context.Context, client clients.CloudStorage, outputDi
 
 	for _, eco := range ecosystems {
 		vulns := entries[eco]
-		// Sort by LastModified descending
+		// Sort by LastModified ascending (oldest first)
 		sort.Slice(vulns, func(i, j int) bool {
-			return vulns[i].LastModified.After(vulns[j].LastModified)
+			return vulns[i].LastModified.Before(vulns[j].LastModified)
 		})
 
 		// Split into chunks
@@ -258,7 +258,8 @@ func generateSitemaps(ctx context.Context, client clients.CloudStorage, outputDi
 			// Add to index
 			chunkLastMod := time.Unix(0, 0).UTC()
 			if len(chunk) > 0 {
-				chunkLastMod = chunk[0].LastModified
+				// Since we sort by oldest first, the last item is the newest
+				chunkLastMod = chunk[len(chunk)-1].LastModified
 			}
 
 			sitemapIndexEntries = append(sitemapIndexEntries, SitemapIndexEntry{
diff --git a/go/cmd/generatesitemap/generatesitemap_test.go b/go/cmd/generatesitemap/generatesitemap_test.go
index 3885f8b637a..7623f4a4250 100644
--- a/go/cmd/generatesitemap/generatesitemap_test.go
+++ b/go/cmd/generatesitemap/generatesitemap_test.go
@@ -111,9 +111,9 @@ func TestGenerateSitemaps(t *testing.T) {
 	if len(urlSet.URLs) != 2 {
 		t.Errorf("expected 2 URLs in sitemap_Go.xml, got %d", len(urlSet.URLs))
 	}
-	// Verify sorting (descending)
-	if urlSet.URLs[0].Loc != "https://example.com/vulnerability/GO-2" {
-		t.Errorf("expected GO-2 first (newer), got %s", urlSet.URLs[0].Loc)
+	// Verify sorting (ascending)
+	if urlSet.URLs[0].Loc != "https://example.com/vulnerability/GO-1" {
+		t.Errorf("expected GO-1 first (older), got %s", urlSet.URLs[0].Loc)
 	}
 
 	// Verify content of sitemap_index.xml