Ranil Wickremesinghe's move clears the way for new President Gotabaya Rajapaksa.
Sri Lanka's Prime Minister Ranil Wickremesinghe formally announced his resignation on Wednesday, four days after his party suffered a humiliating defeat at presidential polls.
In an address to the nation, Mr Wickremesinghe said he had informed new president Gotabaya Rajapaksa that he will step down Thursday, allowing a new minority government to be formed.