Colombo: UNF leader Ranil Wickremesinghe on Friday took oath as Sri Lanka's prime minister at a ceremony at the presidential secretariat here.
Wickremesinghe took oath before President Maithripala Sirisena, reports Xinhua news agency.
This is the fourth time Wickremesinghe is becoming the prime minister of the island nation. He led the United National Front to a victory in Monday's parliamentary elections.
His party defeated the United People's Freedom Alliance (UPFA) by winning 106 seats in the 225-member parliament. The UPFA won 95 seats.
After the ceremony, the Sri Lanka Freedom Party (SLFP), the biggest coalition partner in the UNF, said it would form a united government.
The SLFP, headed by President Sirisena, has said on Thursday that it would join the UNF for two years. And if the united government was successful, it would continue with the experiment.
"If not, we will withdraw at the end of two years," SLFP senior member A.H.M. Fowzie said.
Wickremesinghe ranked first in the preferential votes list by winning more than 500,000 votes from Colombo district.
Former president Mahinda Rajapakse attended Friday's ceremony. He has said he will remain an MP and sit in the opposition benches.
Sri Lanka's new parliament is expected to convene on September 1.