A sponsor bank, also called a partner bank or BaaS bank, is a chartered, insured depository institution that partners with a fintech or a banking-as-a-service (BaaS) platform so the fintech can offer deposit accounts, cards, payments, or lending without holding a bank charter itself. Only a chartered bank can hold a master account at the Federal Reserve, reach ACH/wire/card rails directly, and offer FDIC deposit insurance, so fintechs get that infrastructure by partnering with a sponsor bank instead of pursuing its own multi-year, capital-intensive chartering process.