This pre-registered study aims to address a critical yet underexplored question: To what extent does board gender diversity, specifically the presence of women on boards (WOB), influence corporate dividend payout policies within a bank-based (financial) system? This report further plans to examine how dividend payouts are affected by bank–firm relationships. In particular, it investigates whether WOB function as a substitute for, or complement to, bank monitoring in enhancing dividend payouts in a bank-based system. The anticipated findings are expected to contribute to policy discourse surrounding Japan's Corporate Governance Code, which intends to promote greater board gender diversity, and to the broader debate concerning “Who are effective monitors in influencing corporate payout behavior within bank-based systems?”