Millions of Sri Lankans had voted for the opposition parliamentarian, putting faith in his graft-fighting pledge and vow to bolster a fragile economic recovery.
"I pledge ... to demonstrate dedication to protecting and upholding democracy," Dissanayake, 55, said in inauguration remarks at the president's office, saying he was taking office at a challenging time in the Indian Ocean nation, Reuters reported.
"Our politics needs to be cleaner, and the people have called for a different political culture," he added. "I am ready to commit to that change."
Dissanayake ran as the candidate for the National People's Power (NPP) alliance, which includes his Janatha Vimukhti Peremuna (JVP) party that traditionally championed Marxist economic policies centered on protectionism and state intervention.
In recent years the party has taken more centrist positions.
Tasks he now faces include setting up a new cabinet and wooing parliament, where his party has just three of 225 seats, to pass a budget under the terms of a $2.9-billion bailout from the International Monetary Fund (IMF).
Dissanayake was declared the new president late on Sunday, displacing Wickremesinghe, who had been appointed by parliament to serve out Rajapaksa's remaining term.
"China hopes that Sri Lanka will maintain its national stability and development, and is willing to play a constructive role in the smooth economic and social development," a Chinese foreign ministry spokesperson said on Monday.
The spokesperson declined to give details of the prospect of Sri Lanka wanting to revisit debt agreements it struck with Beijing but said China hoped to deepen high-quality development in the construction of the Belt and Road together.
SD/