diff --git a/functions/src/scripts/backfill-group-ids.ts b/functions/src/scripts/backfill-group-ids.ts
new file mode 100644
index 00000000..ddce5d99
--- /dev/null
+++ b/functions/src/scripts/backfill-group-ids.ts
@@ -0,0 +1,25 @@
+// We have some groups without IDs. Let's fill them in.
+
+import * as admin from 'firebase-admin'
+import { initAdmin } from './script-init'
+import { log, writeAsync } from '../utils'
+
+initAdmin()
+const firestore = admin.firestore()
+
+if (require.main === module) {
+  const groupsQuery = firestore.collection('groups')
+  groupsQuery.get().then(async (groupSnaps) => {
+    log(`Loaded ${groupSnaps.size} groups.`)
+    const needsFilling = groupSnaps.docs.filter((ct) => {
+      return !('id' in ct.data())
+    })
+    log(`${needsFilling.length} groups need IDs.`)
+    const updates = needsFilling.map((group) => {
+      return { doc: group.ref, fields: { id: group.id } }
+    })
+    log(`Updating ${updates.length} groups.`)
+    await writeAsync(firestore, updates)
+    log(`Updated all groups.`)
+  })
+}