The International Atomic Energy Agency's so-called safeguards agreement is meant to allow monitors oversight of 14 Indian civilian nuclear reactors by 2014. Six of these reactors already fall under existing agreements.
Monday's signing of the deal by the chief U.N. nuclear inspector, IAEA Director General Mohamed ElBaradei, and by Indian Ambassador Saurabh Kumar was expected.
It came half a year after the IAEA's 35-member board of governors approved it by consensus on Aug. 1, despite criticism from some countries and experts that ambiguous wording could limit international oversight of India's reactors and undermine the international nonproliferation treaty. Some also warned it could help supply India's arms programs with fissile material.
At the time, ElBaradei hailed the agreement as good for both India and the world.
The agreement will enter into force once India notifies the Vienna-based IAEA in writing that it has ratified it. It was not immediately clear when that would be.
The IAEA-India inspections plan also was one of the prerequisites for implementation of the landmark U.S.-India nuclear accord that was signed on Oct. 10.
It permits U.S. businesses to sell nuclear fuel, technology and reactors to India and reverses a three-decade ban on atomic trade with the fast-growing nuclear-armed Asian power.
The U.S.-Indian civilian nuclear cooperation agreement, the result of three years of often frustrating political and diplomatic wrangling, marked a major shift in U.S. policy toward India after decades of mutual wariness.
India had faced a nuclear trade ban since its first atomic test in 1974 and has refused to sign nonproliferation accords.
Pakistan, India's neighboring nuclear rival and opponent in three wars since their independence from Britain in 1947, was vocal in its opposition to the safeguards deal in the weeks leading up to its consideration in August.
But Islamabad did not oppose the consensus approval.