Modi also said he looks forward to working closely with Mahinda on further strengthening fraternal India-Sri Lanka ties.
"Congratulations and best wishes to Prime Minister Mahinda Rajapaksa. I look forward to working closely with him for further strengthening fraternal India-Sri Lanka ties," PM Modi tweeted.
Mahinda, who is a brother of newly elected President Gotabaya Rajapaksa, was sworn in as the new prime minister of the island nation.
This is the first time in the history of Sri Lanka that two siblings are in the positions of president and the prime minister.
Mahinda Rajapaksa has served as president from 2005 to 2015. His accession to power has come after Ranil Wickremesinghe announced his resignation from the prime ministerial post, paving a way for a caretaker cabinet until the general elections which can be held in March 2020.