diff --git a/h/migrations/versions/f32200e2e496_backfill_user_pubid.py b/h/migrations/versions/f32200e2e496_backfill_user_pubid.py
new file mode 100644
index 00000000000..2d74d447ced
--- /dev/null
+++ b/h/migrations/versions/f32200e2e496_backfill_user_pubid.py
@@ -0,0 +1,76 @@
+"""Backfill user.pubid with unique value."""
+
+import logging
+import random
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.orm import declarative_base, sessionmaker
+
+revision = "f32200e2e496"
+down_revision = "3bae642f2b36"
+
+logger = logging.getLogger(__name__)
+
+USER_PUBID_LENGTH = 12
+USER_BATCH_LIMIT = 1000
+USER_PUBID_RETRIES = 5
+
+
+Base = declarative_base()
+
+
+class User(Base):
+    __tablename__ = "user"
+    id = sa.Column(sa.Integer, primary_key=True)
+    pubid = sa.Column(sa.String())
+
+
+def generate(length):
+    """
+    Generate a random string of the specified length.
+
+    This is the generate() function from h/pubid.py.
+    """
+    alphabet = "123456789ABDEGJKLMNPQRVWXYZabdegijkmnopqrvwxyz"
+    return "".join(random.SystemRandom().choice(alphabet) for _ in range(length))
+
+
+def get_unique_pubids(count, length):
+    return {generate(length) for _ in range(count)}
+
+
+def set_user_pubids(users, pubids):
+    for user, pubid in zip(users, pubids):
+        user.pubid = pubid
+
+
+def upgrade():
+    session = sessionmaker()(bind=op.get_bind())
+
+    users_query = (
+        session.query(User).filter(User.pubid.is_(None)).limit(USER_BATCH_LIMIT)
+    )
+
+    count = 0
+    while users := users_query.all():
+        batch_count = len(users)
+        for _ in range(USER_PUBID_RETRIES):
+            pubids = get_unique_pubids(batch_count, USER_PUBID_LENGTH)
+            if len(pubids) == batch_count:
+                break
+            logger.warning("Failed to generate %d unique pubids, retrying", batch_count)
+        else:
+            raise RuntimeError(f"Failed to generate {batch_count} unique pubids")
+
+        set_user_pubids(users, pubids)
+        session.commit()
+        count += batch_count
+        logger.info("Back-filled %d user.pubid's", count)
+
+
+def downgrade():
+    session = sessionmaker()(bind=op.get_bind())
+
+    session.query(User).update({User.pubid: None})
+    session.commit()