Rajapaksa, who contested the presidential election from the Sri Lanka Podujana Peramuna (SLPP) party, was declared as the President on Sunday upon his victory, the Daily Mirror reported.
The swearing-in ceremony took place in Ruwanwelisaya locate in the ancient Buddhist city of Anuradhapura.
SLPP and Opposition Leader Mahinda Rajapaksa, party Chairman G.L. Peiris, National Organiser Basil Rajapaksa, Prime Minister Ranil Wickremesinghe along with other political leaders attended the swearing-in ceremony.
Gotabaya won the majority Sinahala votes of 6,924,255, or 52.25 per cent, from a total valid 13,252,499 ballots cast in the November 16 election.
The former Defence Secretary in the government of his brother Mahinda Rajapaksa, oversaw victory in the 26-year-long war against rebel Liberation Tigers of Tamil Eelam (LTTE) in the North and East in 2009.
Out of 25 administrative districts, Gotabaya won 16: Kalutara, Galle, Matara, Hambantota, Monaragala, Ratnapura, Badulla, Kurunegala, Puttalam, Gampaha, Kandy, Matale, Polonnaruwa, Colombo, Kegalle and Anuradhapura.
However, he was heavily defeated in Tamil and Muslim-majority Northern and Eastern Provinces. The Tamils in all five administrative districts in the Northern province Jaffna, Kilinochchi, Mulaitivu, Vavuniya, Mannar, and the majority Muslims in three districts in the Eastern Province Trincomalee, Batticaloa, Ampara voted against Rajapaksa with lead of 80 percent in most districts.
The November 16 election was the third since the end of the civil war and the first after the April 21 Easter Sunday bombings which killed over 250 people this year.
( With inputs from IANS )