"I'm honoured to have taken oaths as Sri Lanka's new prime minister", Mahinda Rajapaksa tweeted.
He urged Prime Minister Narendra Modi and External Affairs Minister S. Jaishankar to draw the attention of the new Sri Lanka President to ensure punishments for those involved in war crimes, and also to fulfil the promises given to the UN Human Rights Commission, during the scheduled meetings.
The swearing in ceremony is taking place at the Presidential secretariat and can be viewed live here.
This is the first time in the history of Sri Lanka that two siblings are in the positions of president and the prime minister.
The Sri Lankan premier thanked Prime Minister Imran for his felicitations and said that he looks forward to working together to further strengthen Sri Lanka-Pakistan relations.
Yesterday, he delivered a special statement to the media, announcing that he would resign from his post as the Prime Minister of Sri Lanka.
Gotabaya, 70, had earlier served as defence secretary under his brother Mahinda, who himself served as president from 2005-2015.
He is set to be his party's candidate in a parliamentary election expected around April.
The two brothers - Mahinda and Gotabhaya - led a decisive campaign that helped end the island nation's three decade long civil war against the Liberation Tigers of Tamil Eelam (LTTE).
"The Rajapaksa brand, as evidenced by the election, remains extremely strong", said Akhil Bery, Eurasia's South Asia analyst, adding that Mahinda is perceived to be even more popular than his younger brother.
Gotabaya, who oversaw the defeat of Tamil separatists as his brother's defence chief a decade ago, won the election after promising to secure the country against militant threats.
But the Rajapaksas have denied any wrongdoing.