Gunawardena, 75, took over as prime minister in July 2022 after former President Gotabaya Rajapaksa fled the country and resigned in the face of widespread protests unleashed by the worst economic crisis in decades.
Sunday's election saw Dissanayake defeat incumbent Ranil Wickremesinghe, who had been appointed by parliament to serve out Rajapaksa's remaining term.
"I respectfully inform you that I hereby resign as the prime minister," Gunawardena said in a post on X, thanking Wickremesinghe for support extended during his tenure.
Media said Gunawardena had already sent his resignation to President-elect Dissanayake
