Former President Mahinda Rajapaksa took oaths as the Prime Minister before President Gotabhaya Rajapaksa.
This is former President Rajapaksa's third stint as the Prime Minister.
The swearing-in ceremony was attended by a number of political leaders, including former Prime Minister Ranil Wickremesinghe.
President Gotabhaya Rajapaksa is expected to appoint a 15-member cabinet to administer the government until the next Parliamentary election.