The role will be in addition to his duties as deputy first minister and cabinet secretary for Covid recovery.
Ms Forbes’ maternity leave is due to begin on July 16, but she will still hold the title of cabinet secretary for finance and economy for the duration of her leave.
A date for her return has not yet been confirmed
Mr Swinney, who has previously served as finance secretary and education secretary, said: “This is a really exciting time for Kate and everyone is incredibly happy about her news.”
He said he is “aware of the importance of the role”, adding: “It is a real honour to take it on.”
Earlier in the year, First Minister Nicola Sturgeon said having a baby “should never be a barrier to someone’s career, especially women in senior leadership roles,” and confirmed Mr Forbes is welcome to return to her post.
Ms Forbes said: “I am looking forward to the new arrival as my husband Ali and I welcome our baby into the world.”
She added: “John Swinney’s vast experience in this brief means there is no-one better suited to taking on this role.
“I know that, during my time away, he will continue to drive forward this government’s ambitious programme of economic transformation and support for our public services.”
Ms Forbes became the first female finance secretary in 2020 after Derek Mackay resigned.
