"There being no other proposal, I declare Sonia Gandhi unanimously elected as leader of the Congress Parliamentary Party," said senior Congress leader Manmohan Singh.
Congress has the largest number of MPs following its surprise routing of the ruling Hindu nationalists in India's marathon elections that ended on 10 May.
The election of Gandhi as Congress parliamentary chief is effectively the party's nomination for the post of prime minister.
"Truly this is a historic moment," said Singh, referring to the period Congress has been sitting in opposition benches. "A long spell of cold winter of nearly nine years is over and now we are in bright sunshine."
Gandhi has been non-committal about whether she wants to become the fourth member of the Nehru-Gandhi dynasty to hold the post of prime minister.
In meetings over the next few days, Congress and its allies will arrive at a consensus over who should head the coalition government.
"Truly this is a historic moment. A long spell of cold winter of nearly nine years is over and now we are in bright sunshine"
senior Congress leader
That consensus candidate will then ask President Abd al-Kalam for permission to form a new government for the country of a billion-plus people.
Speaking to her MPs after the vote, Gandhi said: "Very shortly a Congress-led coalition government will be in place at the centre."