diff --git a/node-src/tasks/gitInfo.test.ts b/node-src/tasks/gitInfo.test.ts
index cab1e6d96..ba122dbe7 100644
--- a/node-src/tasks/gitInfo.test.ts
+++ b/node-src/tasks/gitInfo.test.ts
@@ -154,4 +154,14 @@ describe('setGitInfo', () => {
     expect(ctx.rebuildForBuild).toEqual(lastBuild);
     expect(ctx.storybookUrl).toEqual(lastBuild.storybookUrl);
   });
+
+  it('removes undefined owner prefix from branch', async () => {
+    const ctx = { log, options: {}, client } as any;
+    getCommitAndBranch.mockResolvedValue({
+      ...commitInfo,
+      branch: 'undefined:repo',
+    });
+    await setGitInfo(ctx, {} as any);
+    expect(ctx.git.branch).toBe('repo');
+  });
 });
diff --git a/node-src/tasks/gitInfo.ts b/node-src/tasks/gitInfo.ts
index a4e6eb843..1772c1d4c 100644
--- a/node-src/tasks/gitInfo.ts
+++ b/node-src/tasks/gitInfo.ts
@@ -15,6 +15,7 @@ import replacedBuild from '../ui/messages/info/replacedBuild';
 import externalsChanged from '../ui/messages/warnings/externalsChanged';
 import invalidChangedFiles from '../ui/messages/warnings/invalidChangedFiles';
 import isRebuild from '../ui/messages/warnings/isRebuild';
+import undefinedBranchOwner from '../ui/messages/warnings/undefinedBranchOwner';
 import {
   initial,
   pending,
@@ -25,6 +26,8 @@ import {
   success,
 } from '../ui/tasks/gitInfo';
 
+const UNDEFINED_BRANCH_PREFIX_REGEXP = /^undefined:/;
+
 const SkipBuildMutation = `
   mutation SkipBuildMutation($commit: String!, $branch: String, $slug: String) {
     skipBuild(commit: $commit, branch: $branch, slug: $slug)
@@ -99,6 +102,10 @@ export const setGitInfo = async (ctx: Context, task: Task) => {
   if (ownerName) {
     ctx.git.branch = ctx.git.branch.replace(/[^:]+:/, '');
     ctx.git.slug = repositorySlug || ctx.git.slug?.replace(/[^/]+/, ownerName);
+  } else if (UNDEFINED_BRANCH_PREFIX_REGEXP.test(ctx.git.branch)) {
+    // Strip off `undefined:` owner prefix that we have seen in some CI systems.
+    ctx.log.warn(undefinedBranchOwner());
+    ctx.git.branch = ctx.git.branch.replace(UNDEFINED_BRANCH_PREFIX_REGEXP, '');
   }
 
   const { branch, commit, slug } = ctx.git;
diff --git a/node-src/ui/messages/warnings/undefinedBranchOwner.stories.ts b/node-src/ui/messages/warnings/undefinedBranchOwner.stories.ts
new file mode 100644
index 000000000..ad817360f
--- /dev/null
+++ b/node-src/ui/messages/warnings/undefinedBranchOwner.stories.ts
@@ -0,0 +1,5 @@
+export default {
+  title: 'CLI/Messages/Warnings',
+};
+
+export { default as UndefinedBranchOwner } from './undefinedBranchOwner';
diff --git a/node-src/ui/messages/warnings/undefinedBranchOwner.ts b/node-src/ui/messages/warnings/undefinedBranchOwner.ts
new file mode 100644
index 000000000..4fc82238f
--- /dev/null
+++ b/node-src/ui/messages/warnings/undefinedBranchOwner.ts
@@ -0,0 +1,16 @@
+import chalk from 'chalk';
+import { dedent } from 'ts-dedent';
+
+import { info, warning } from '../../components/icons';
+import link from '../../components/link';
+
+const docsUrl =
+  'https://www.chromatic.com/docs/custom-ci-provider/#overriding-chromatics-branch-detection';
+
+export default () => {
+  return dedent(chalk`
+    ${warning} Removing unknown owner prefix from branch name.
+    You may wish to set the branch directly to avoid incorrect values.
+    ${info} Read more at ${link(docsUrl)}
+  `);
+};