Neither personal nor business customers were able to use the app to transfer money or view balances.
Users saw an error message as they tried to log on that asked them to check back later, and giving a number to ring.
One customers complained the app was “flaky at best”, others threatened to move their accounts.
Keith Mortiss tweeted: “Mobile banking app not working AGAIN – sort this out HSBC, before I move my accounts.”
Sarah Drayton, said the app had been “playing up” all day, and Mary Birch said the failure was distressing.
Many users tweeted to the bank to ask what the problem was.
HSBC apologised and said it was working hard to fix the outage.
The HSBC app has suffered earlier failures – in September and on Black Friday in November – the busiest shopping day of the year – as well as earlier this month.
MPs on the Treasury committee are investigating IT failures in the financial services sector following a string of technical meltdowns at high street banks.