diff --git a/src/setup-gcloud.ts b/src/setup-gcloud.ts
index d70ba3e32..0fde61f67 100644
--- a/src/setup-gcloud.ts
+++ b/src/setup-gcloud.ts
@@ -40,7 +40,36 @@ export const GCLOUD_METRICS_LABEL = 'github-actions-setup-gcloud';
 export async function run(): Promise<void> {
   // Warn if pinned to HEAD
   if (isPinnedToHead()) {
-    core.warning(pinnedToHeadWarning('v0'));
+    // TODO(sethvargo): switch back to warning level after branch is renamed
+    // from "master" -> "main".
+    core.error(pinnedToHeadWarning('v0'));
+
+    // TODO(sethvargo): remove after branch is renamed from "master" -> "main".
+    const envvar =
+      'SETUP_GCLOUD_I_UNDERSTAND_USING_MASTER_WILL_BREAK_MY_WORKFLOW_ON_APRIL_5_2022';
+    if (process.env.GITHUB_ACTION_REF === 'master' && !process.env[envvar]) {
+      throw new Error(
+        `On 2022-04-05, the default branch will be renamed from "master" to ` +
+          `"main". Your action is currently pinned to "@master". Even though ` +
+          `GitHub creates redirects for renamed branches, testing found that ` +
+          `this rename breaks existing GitHub Actions workflows that are ` +
+          `pinned to the old branch name.\n` +
+          `\n` +
+          `We strongly advise updating your GitHub Action YAML from:\n` +
+          `\n` +
+          `    uses: 'google-github-actions/setup-gcloud@master'\n` +
+          `\n` +
+          `to:\n` +
+          `\n` +
+          `    uses: 'google-github-actions/setup-gcloud@v0'\n` +
+          `\n` +
+          `While not recommended, you can still pin to the "master" branch ` +
+          `by setting the environment variable "${envvar}=1". However, on ` +
+          `2022-04-05 when the branch is renamed, all your workflows will ` +
+          `begin failing with an obtuse and difficult to diagnose error ` +
+          `message.`,
+      );
+    }
   }
 
   core.exportVariable(GCLOUD_METRICS_ENV_VAR, GCLOUD_METRICS_LABEL);