Mr Wickremesinghe was sworn in by President Maithripala Sirisena, who sacked him on October 26, report agencies.
United National Party leader Ranil Wickremesinghe was sworn in as the Prime Minister of Sri Lanka on Sunday, ending a 51-day power tussle in the island nation that had crippled the government.
Mr. Wickremesinghe was administered the oath of office by President Maithripala Sirisena at a simple ceremony held at the Presidential Secretariat in Colombo.
His appointment comes hours after former strongman Mahinda Rajapaksa, who was appointed Prime Minister by President Maithripala Sirisena, resigned on Saturday, paving way for the return of Wickremesinghe as premier.
The South Asian island country had plunged into instability after Sirisena replaced Wickremesinghe with Mahinda Rajapaksa, who was then twice sacked by parliament. Rajapaksa resigned on Saturday as a government shutdown loomed.