diff --git a/.changeset/strong-insects-provide.md b/.changeset/strong-insects-provide.md
new file mode 100644
index 0000000000..1a271b220b
--- /dev/null
+++ b/.changeset/strong-insects-provide.md
@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-catalog-backend': patch
+---
+
+Filter out projects with missing `default_branch` from GitLab Discovery.
diff --git a/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.test.ts b/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.test.ts
index 311de05673..8d85d7375f 100644
--- a/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.test.ts
+++ b/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.test.ts
@@ -184,6 +184,13 @@ describe('GitlabDiscoveryProcessor', () => {
                   last_activity_at: '2021-08-05T11:03:05.774Z',
                   web_url: 'https://gitlab.fake/3',
                 },
+                {
+                  id: 4,
+                  archived: false,
+                  default_branch: undefined, // MISSING DEFAULT BRANCH
+                  last_activity_at: '2021-08-05T11:03:05.774Z',
+                  web_url: 'https://gitlab.fake/4',
+                },
               ],
             };
           default:
diff --git a/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.ts b/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.ts
index 70ccd44793..fabf3a1136 100644
--- a/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.ts
+++ b/plugins/catalog-backend/src/ingestion/processors/GitLabDiscoveryProcessor.ts
@@ -94,9 +94,16 @@ export class GitLabDiscoveryProcessor implements CatalogProcessor {
     };
     for await (const project of projects) {
       result.scanned++;
-      if (!project.archived) {
-        result.matches.push(project);
+
+      if (project.archived) {
+        continue;
       }
+
+      if (branch === '*' && project.default_branch === undefined) {
+        continue;
+      }
+
+      result.matches.push(project);
     }
 
     for (const project of result.matches) {
diff --git a/plugins/catalog-backend/src/ingestion/processors/gitlab/types.ts b/plugins/catalog-backend/src/ingestion/processors/gitlab/types.ts
index a66411ddc8..d41ed73aac 100644
--- a/plugins/catalog-backend/src/ingestion/processors/gitlab/types.ts
+++ b/plugins/catalog-backend/src/ingestion/processors/gitlab/types.ts
@@ -16,7 +16,7 @@
 
 export type GitLabProject = {
   id: number;
-  default_branch: string;
+  default_branch?: string;
   archived: boolean;
   last_activity_at: string;
   web_url: string;