Anura Kumara Dissanayake won with over 40 percent of the votes, placing opposition leader Sajith Premadasa in second place.
The incumbent president Ranil Wickremesinghe placed third, far behind.
Anura Kumara Dissanayake quickly took the lead – and Sajith Premadasa had already declared himself defeated earlier.
About 13 million of the approximately 17 million eligible voters have exercised their right to vote, according to an election survey.
Ranil Wickremesinghe has been president since 2022. He was elected by parliament after the elected president Gotabaya Rajapaksa was forced to flee the country following riots and large-scale protests against the difficult, Christian living conditions prevailing in the country after economic mismanagement and downturn during the pandemic.
The election campaign has largely focused on the economy and the terms for emergency loans from the International Monetary Fund (IMF).