"In keeping with the excellent cooperation between India and the United States to combat terrorism, we offer you our full support," the prime minister wrote in a letter to Obama.
He expressed solidarity with the American people in the struggle against terrorism. "This senseless and cowardly act of violence has struck a city that has long stood as a symbol of openness, learning and enterprise."
Manmohan Singh added: "The people of India join me in condemning the attack in strongest terms."
At least three people were killed and 141 injured as two powerful explosions rocked the Boston Marathon finish line Monday afternoon.
The dead included an eight-year-old boy. Boston Children's Hospital reported that those treated there included a nine-year-old girl, a seven-year-old boy, a 12-year-old, and a two-year-old.