Prime Minister Narendra Modi congratulated Rajapaksa and said he looks forward to further deepen relations between the two nations.
Sri Lanka's controversial wartime defence secretary Gotabaya Rajapaksa will be the next President after ruling party candidate Sajith Premadasa conceded the hotly contested election on Sunday, marking the return of the powerful Rajapaksa dynasty known for its pro-China tilt, amidst security challenges following the devastating Easter Sunday terror attacks that killed 269 people.
Rajapaksa, 70, defeated Premadasa, 52, by more than 13 lakh votes, according to the official results.
Rajapaksa, who will succeed President Maithripala Sirisena for a five-year term, will be sworn in as the seventh executive president of Sri Lanka on Monday at the ancient north central town of Anuradhapura.
He will be the second member from the Rajapaksa family to become the president. His older brother Mahinda Rajapaksa was president from 2005 to 2015.
Rajapaksa secured 52.25 per cent votes (6,924,255) while Premadasa received 41.99 per cent of the total votes polled (5,564,239), the election commission said.
The overall voter turnout at the election was around 83.73 per cent, Elections Commission Chairman Mahinda Deshapriya said.
Following his win, Rajapaksa urged his supporters to 'rejoice peacefully'.
'As we usher in a new journey for Sri Lanka, we must remember that all Sri Lankans are part of this journey. Let us rejoice peacefully, with dignity and discipline in the same manner in which we campaigned,' the retired lieutenant colonel said in a tweet.
Rajapaksa had vowed to 'restore relations' with Sri Lanka's top lender, China, if he wins the election, despite international concerns over the island nation's financial debt to the Asian superpower.
Premadasa, meanwhile, was considered leaning more towards India and the US.
He conceded the presidential election to his rival even before the official results were announced and also stepped down as the Deputy Leader of the ruling United National Party (UNP) with immediate effect.
'It is my privilege to honour the people's decision and congratulate Mr Gotabaya Rajapaksa on his election,' Premadasa said.
'I am grateful our citizens who voted for me. I am humbled that you placed your faith in me. Your support has been a fountain of strength throughout my political career,' he said on Twitter.
Rajapaksa, who was the top defence ministry bureaucrat for 10 years, swept the poll in the Sinhala majority districts while Premadasa garnered most of the votes from the Tamil dominated north and east and Muslim community, including the tea plantation workers of Indian origin.
Prime Minister Narendra Modi congratulated Rajapaksa and said he looks forward to further deepen relations between the two nations.
Rajapaksa thanked the people of India and Modi, saying he looks forward to strengthening the friendship and meeting him in the near future.
Pakistan President Arif Alvi and his Maldivian counterpart Ibrahim Mohamed Solih also congratulated Rajapaksa.
Meanwhile, Prime Minister Ranil Wickremesinghe, who cannot be removed unless he steps down, is expected to resign.
It is believed that Rajapaksa, after taking over as the President, would appoint his elder brother and former strongman Mahinda as the Prime Minister.
Mahinda in a statement said one of the first tasks of the new government is to study the 19th amendment to the constitution adopted in 2015.
The 19A introduced a three-time bar to contest the presidency and made key areas such as elections, judiciary and public service independent of political influence.
Rajapaksa senior took to Twitter to congratulate his younger brother on election victory.
'The long campaign trail to serve our motherland again is worth it,' he said.
Congratulating Rajapaksa, outgoing President Sirisena termed his win as 'historic'.
Premadasa, the son of assassinated president Ranasinghe Premadasa, is a seasoned politician with an experience of more than 25 years in the power corridors.
Mahinda's legacy of ending the Tamil separatist war has made Rajapaksa the darling of the Sinhala Buddhist majority. Gotabhaya was his top defence ministry official who supervised the military operations against the Liberation Tigers of Tamil Elam.
While doing so, he acquired the reputation of a ruthlessly efficient administrator.
Rajapaksa's victory marks the return of the family to the pinnacle of power in the island after they were surprisingly ousted in the previous presidential poll.
According to observers, with pro-China Rajapaksa winning the election, the result will have a bearing on India's presence in the Indian Ocean region where Beijing is increasingly making inroads.
China, which has acquired Sri Lanka's Hambantota port in 2017 as a debt swap, has been ramping up its ties with the island nation and expanded its naval presence in the Indian Ocean with an established logistics base in Djibouti.
Beijing in July gifted a warship to Sri Lanka, in a growing sign of its deepening military cooperation with the strategically located island nation in the Indian Ocean.
The Lanka election took place nearly seven months after the Easter Sunday attacks that killed 258 people, seriously hitting the tourism industry, one of the main forex earning sectors of the country.
Gotabhaya is being seen as a leader who is most trusted to safeguard national security after the Easter Sunday bombings.
