The chancellor announced a £500m package to support mental health services in England after increased demand for support during the pandemic.
The Treasury said the majority of the funding will be spent on specialist services for young people, including in schools, and support for NHS workers.
Mr Sunak is also expected to vow for rapid progress to tackle the backlog of adult mental health referrals.
The government estimates that mental ill health costs the economy up to £35bn per year, and hopes the package will address the extra demand for services amid the pandemic.
Mr Sunak said the pandemic has had “a major impact on mental health because of increased isolation and uncertainty”.
The package will also be spent on extra support for people with severe mental illness, and faster access to psychological support for conditions such as depression and anxiety, the Treasury said.
Local areas will benefit from £1.2m for a new service to allow them to access real-time suicide data to better target prevention efforts, it added.