Ousted in January, the former president aims to return to power as premier and is a polarising figure.
He was hugely popular with the majority Sinhalese for crushing Tamil separatist rebels in a no-holds-barred offensive in 2009 that ended a 37-year-long ethnic war. But the Tamil minority voted overwhelmingly for his successor in January. He has been criticised for filling positions of power with relatives, some of whom face corruption charges.
His success in overseeing an end to the war gave him a big election win and a second term in office, during which he removed a two-term limit on the presidency and gave himself more powers. But his move to call a snap election for a third term led to a revolt within his own party.
He publicly blamed his defeat on lack of support from Tamils and Muslims, who together account for about a quarter of Sri Lanka's population of 20 million and can tip the scales when the majority Sinhalese are divided.
The new government has launched investigations into charges that Mr Rajapaksa and his family siphoned off billions of dollars. A lawyer by profession, he is a strong believer in astrology. In the current campaign, he was dubbed Lord of the Rings because he wore rings as lucky charms - adding to the number as the campaign got tougher.
He was a relative unknown even in Sri Lanka until he became the opposition's surprise choice to challenge Mr Mahinda Rajapaksa for the top job.
The 63-year-old became an unlikely rallying point for disaffected Sri Lankans after walking out of Mr Rajapaksa's government.
He promised radical reforms, including abolishing the executive presidency within 100 days and returning the country to a parliamentary democracy. His legislative agenda was stalled by MPs loyal to Mr Rajapaksa.
He has succeeded in introducing some reforms, depoliticising the police, the judiciary and other public services, and restoring a two-term limit on the presidency.
President Sirisena has been vocal about his predecessor.
"If I had not won the presidency, I would be six feet under by now," he said at a public rally after his win, accusing the former leader of plotting to assassinate him and his family if he had lost.
Mr Sirisena addresses international and regional gatherings in his native Sinhalese, although he is competent in English.
He appeals to a rural electorate, while his main backer, the centre-right United National Party (UNP), is more popular in urban areas.
His vision for the country ties in with the free-market, investor-friendly policies of the UNP, which gave him the political base to challenge Mr Rajapaksa.
The reformist prime minister is a champion of free enterprise who has won support from both the West and India during his eight months in office.
The 66-year-old was handpicked by Mr Maithripala Sirisena to lead a minority government after the ousting of Mr Rajapaksa in January.
His United National Party is topping polls, so he could have a shot at a fourth stint as premier.
He became premier in 1993, when a suicide bomber killed president Ranasinghe Premadasa.
In 1999, he lost the presidential race after Ms Chandrika Kumaratunga, who hails from the country's famed Bandaranaike clan, was wounded in a suicide bomb attack.
She went on to win narrowly on a wave of sympathy.
Mr Wickremesinghe got another chance in 2002, when he was credited with pulling the country out of its first recession and boosting economic growth.
In his campaign for a fourth term in office, he has pledged to overhaul Sri Lanka's human rights record and create a "brand new country in 60 months".
Mr Rajapaksa resisted calls by the United Nations to investigate allegations that more than 40,000 Tamil civilians were killed in a 2009 military crackdown on separatist guerrillas, but Mr Wickremesinghe has taken a more accommodating stance, pledging to address the issue.