Dissanayake won about 53 per cent of a million votes counted so far in the election, Sri Lanka's Election Commission data showed on Sunday.
Opposition Leader Sajith Premadasa was second at 22 per cent, ahead of President Ranil Wickremesinghe in third place.
Ranil Wickremesinghe's tenure as Sri Lankan president looks set to end. (AP PHOTO)
About 75 per cent of the eligible 17 million people in the Indian Ocean island nation cast their votes in Saturday's election, according to the poll body.
Dissanayake contested as candidate for the National People's Power (NPP) alliance, which includes his Marxist-leaning Janatha Vimukthi Peremuna (JVP) party that has traditionally backed stronger state intervention, lower taxes  and more closed market economic policies.
Although JVP party has just three seats in parliament, the 55-year-old Dissanayake has been boosted by his promises of tough anti-corruption measures and more pro-poor policies.
He presented himself as the candidate of change, promising to dissolve parliament within 45 days of coming to power in order to seek a fresh mandate for his policies in the general elections.
"After a long and arduous campaign, the results of the election are now clear," Foreign Minister Ali Sabry said on X.
"Though I heavily campaigned for President Ranil Wickremasinghe, the people of Sri Lanka have made their decision, and I fully respect their mandate for Anura Kumara Dissanayake."
Anura Kumara Dissanayake's rousing speeches at rallies have captured the support of the people. (AP PHOTO)
This was Sri Lanka's first election since the economy buckled in 2022 under a severe foreign exchange shortage, leaving the country unable to pay for imports of essentials including fuel, medicine and cooking gas.
Thousands of protesters marched in Colombo in 2022 and occupied the president's office and residence, forcing then-President Gotabaya Rajapaksa to flee and later resign.
Buttressed by a $US2.9 billion ($A4.3 billion) bailout program from the International Monetary Fund, the economy has posted a tentative recovery but the high cost of living was still a critical issue for many voters.
Although inflation cooled to 0.5 per cent last month from a crisis high of 70 per cent, and the economy is forecast to grow in 2024 for the first time in three years.
