The swearing-in took place before Chief Justice Jayantha Jayasuriya in front of a historic Buddhist temple in Anuradhapura, 180 kilometres northeast of the capital.
Gotabaya, the candidate for the Sri Lanka Podujana Party (Peoples’ Party), recorded a comfortable victory with a majority of more than 1.3 million votes in the country’s presidential polls, according to election results.
He received around 52 per cent of the vote, compared to the 42 per cent for his chief rival, ruling party candidate Sajith Premadasa.
Both of the two favourites to win had focussed their campaigns on the theme of security in the wake of this year’s Easter Sunday bombings by extremist Islamist groups that left the entire nation shaken.
President-elect Rajapaksa is the younger brother of former president Mahinda Rajapaksa, who was in power from 2005 to 2015.
Gotabaya Rajapaksa served as defence secretary under his brother and was responsible for directing the military campaign against Tamil rebels, leading to their final defeat in 2009.