It is the first major lender outside state-backed banking groups Royal Bank of Scotland and Lloyds to announce participation in the £12 billion programme.
Taxpayers will guarantee up to 15% of a property's value, in return for a fee charged to lenders, allowing homebuyers to purchase with deposits as low as 5%. Some estimates suggest 180,000 loans could be taken out under the initiative.
Estate agent Haart predicted Help to Buy would boost property transactions by 10-15% in the next 12 months and reduce the average deposit required by first-time buyers from £33,948 to £7,218.
The scheme - brought forward by three months despite fears of a housing bubble - had until now been backed only by brands controlled by RBS and Lloyds, together with smaller players Virgin Money and start-up Aldermore.
The Treasury said these banks represent more than 30% of the market. HSBC said it had a market share of 13.6% of new mortgage approvals in 2012.
Antonio Simoes, head of HSBC UK, said it promises to beat or match rates from high street competitors at 90% LTV.
"By participating in Help to Buy, we will go further and offer mortgages up to 95% LTV," he said.
RBS and its NatWest subsidiary are offering two and five-year fixed-rate deals at 4.99% and 5.49% with no fee and the brands expect to sign up 25,500 first and next-time buyers over the three-year scheme.
The two lenders are extending opening hours at 740 branches for two weeks to cope with expected demand and the Treasury said a flood of interest was expected.
Customers can get information from these banks today and start applying for their Help to Buy mortgages from tomorrow.
At Lloyds-owned Halifax, customers can begin applying from Friday for products including a two-year fixed-rate deal at 5.19% with a £995 product fee. Another Lloyds brand, Bank of Scotland, will also take part, though not Lloyds Bank.
Virgin and Aldermore are due to take part from January and the latter is exploring whether the date can be brought forward, the Treasury said.