71-year-old Babis was sworn in by President Petr Pavel on Tuesday after winning the parliamentary election in October, the AP and AFP news agencies report.
The appointment comes after he promised last week to give up control of the powerful agricultural company Agrofert, so as not to end up in a conflict of interest situation.
Babis, who also led the country from 2017 to 21, is forming a three-party government with his right-wing populist Ano and the far-right SPD and Motorists parties, after promises to, among other things, sharply cut the Czech Republic's support for Ukraine.
Exactly who will take which ministerial posts is not yet entirely clear. President Pavel has had strong objections to several of the Motorists' candidates in particular, who have been accused of both violent crimes and racist and homophobic statements.
