"I congratulate Mr. Antonio Costa on becoming the PM of Portugal. I look forward to working with him to strengthen bilateral ties," he tweeted.
Costa was named Prime Minister on Tuesday at the head of a new Socialist administration, ending more than seven weeks of political turmoil in Portugal after inconclusive elections.
Modi also greeted Mauricio Macri on becoming President of Argentina.
"Greetings to Mr. @mauriciomacri on his poll win. Best wishes as he becomes Argentina's President. India deeply values ties with Argentina," he said in another tweet.