From next spring, nine million HSBC and First Direct customers will be able to access their accounts at 11,500 Post Office branches.
The deal will let personal current account customers make cash withdrawals and deposits, cheque deposits and balance enquiries at Post Office outlets throughout the UK.
HSBC itself has a branch network of 1200, around a tenth of the size of the Post Office's, while the bank's first direct subsidiary has no physical centres, offering its services online, through mobile and over the phone.
Brian Robertson, CEO, HSBC, says: "We recognise that our customers like to bank in a variety of different ways. This new partnership allows us to provide them with an additional opportunity to access their HSBC bank accounts at Post Office counters."
Paula Vennells, chief executive, Post Office, adds: "This new partnership comes as the Post Office embarks on the biggest network investment in its history to both maintain and modernise its unrivalled estate. This agreement means that 95% of all UK debit cardholders will have access to their money at Post Office counters."
The Post Office has also been seeking to increase its own range of banking services in recent years after the previous government decided to push it as an alternative to traditional high street players in the wake of the financial crisis. Last month a major milestone in this strategy was reached with the introduction of in-branch specialist mortgage advisers.