Leavitt will serve as an on-air contributor beginning November 1, the network said on Monday.
"Karoline's firsthand experience in the White House and on the presidential campaign trail gives her a distinct understanding of today's political landscape," said the network's CEO, Suzanne Scott.
For her part, Leavitt, in the same statement, said she looked forward "to bringing my unique perspective from my experience as White House Press Secretary, along with the same energy and candor that defined my tenure, to Fox's millions of viewers across their many platforms".
Leavitt, now 29, was the youngest person to hold the role of press secretary. When she left at the end of August, President Donald Trump described her as one of his "most trusted aides".
At the time she said she wanted to spend more time with her two children. Her second child, Viviana, was born in May, and she has a two-year-old son.
Leavitt is not the first of Trump's press secretaries to join Fox. Kayleigh McEnany joined in 2021 and remains at the network. And Dana Perino, former press secretary for President George W Bush, is a political commentator at the network.
Trump, meanwhile, has hired several Fox News personalities to key jobs in his own administration, chief among them Defence Secretary Pete Hegseth.
Trump has an increasingly thorny relationship with the network, whose key commentators have long given him favourable coverage. He has expressed frustration with Fox coverage choices as recently as Sunday when he bemoaned what he said was the network's lack of coverage of his rallies.
Before becoming White House press secretary, Leavitt served as press secretary for Trump's 2024 presidential campaign.
She also, at age 25, made an unsuccessful bid for Congress in 2022 from her home state of New Hampshire.
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