The US Congress has voted in favour of allowing the export of civilian nuclear fuel and technology to India for the first time in 30 years.
Energy-hungry India needs nuclear power
The legislation will now be sent to President George W Bush to be signed into law.
The vote follows an agreement earlier this year between Mr Bush and the Indian Prime Minister, Manmohan Singh.
The accord has been hailed as historic by some, but critics say it will damage non-proliferation efforts.
It was approved by the House of Representatives on Friday evening and the Senate early on Saturday.
Weapons sites off-limits
Under the deal, energy-hungry India will get access to US civil nuclear technology and fuel, in return for opening its civilian nuclear facilities to inspection.
NUCLEAR POWER IN INDIA
India has 14 reactors in commercial operation and nine under construction
Nuclear power supplies about 3% of India's electricity
By 2050, nuclear power is expected to provide 25% of the country's electricity
India has limited coal and uranium reserves
Its huge thorium reserves - about 25% of the world's total - are expected to fuel its nuclear power programme long-term
Source: Uranium Information Center
But its nuclear weapons sites will remain off-limits.
The Congress decision was welcomed in Delhi as an historic moment in relations between the two countries.
Once on opposite sides of the Cold War fence, they have become allies with close economic, political and even defence ties.
Correspondents say that India sees the deal as a tacit acceptance of its emergence as a global nuclear power.
But some say that by making an exception for India, the US will find it difficult to rein in the nuclear ambitions of North Korea and Iran.
Previously, the US had opposed Indian nuclear activities because it has not signed the Non-Proliferation Treaty (NPT), and has twice tested nuclear weapons, in 1974 and 1998.
The final bill was said to have been altered to take into account some Indian concerns about the deal, says the BBC's Shahzeb Jillani in Washington.
Earlier, senior US state department official Nicholas Burns - who is visiting India - said he anticipated "a very successful and supportive bill", well within the parameters of the agreement signed between India and the US.
India has made clear that the final agreement must not bind it to supporting the US policy on Iran, and does not prevent it from developing its own fissile material.