China also voiced support for the agreement after dropping objections at the last minute in weekend talks in Vienna.
After intense lobbying from Washington and New Delhi, the 45-nation Nuclear Suppliers Group, which controls the export and sale of nuclear technology, approved a waiver of restrictions on trade with India.
It was a vital step in securing a 2005 US-India deal, under which New Delhi will gain access to civilian nuclear technology despite refusing to sign the nuclear Non-Proliferation Treaty (NPT).
The mayors of Hiroshima and Nagasaki, which were destroyed by US nuclear bombs in World War II, both lobbied unsuccessfully for Japan to block the deal.
"We joined the consensus taking a broad viewpoint," said Chief Cabinet Secretary Nobutaka Machimura, the Japanese government's spokesman.
He argued that the deal would help fight global warming.
"It would be a tool to prevent the emission of a huge volume of greenhouse gas as this emerging country of a billion people continues its rapid growth," Machimura said.
Machimura said the deal was a step forward by bringing in international monitoring of Indian nuclear facilities and reinforcing New Delhi's moratorium on nuclear tests.
But he added: "We still hold concerns that the nuclear agreement on India, which is not part of the NPT, may affect the international regime for nuclear arms reduction and non-proliferation."
In Australia, Trade Minister Simon Crean also argued that the deal increased safeguards on India's nuclear programme.
But he insisted that Australia, which has the world's largest proven uranium reserves, was not ready to export to India.
"(The government) shouldn't be changing its policy to not supply to countries that are not signatories to the Non-Proliferation Treaty," Crean said. "India understands that."
Three other countries -- Austria, Ireland and New Zealand -- had been holding out for a clear-cut commitment from India to refrain from nuclear weapons testing but came under strong pressure to agree to the waiver.
Opposition by even one country could have scuttled the deal. The US-India nuclear deal still needs approval by the US Congress, where the clock is ticking ahead of November elections.
China had expressed concerns over lifting the nuclear ban on its neighbour and sometime rival. Under the NPT, China but not India is allowed access to nuclear technology.
China hopes that the Vienna decision "will be conducive to international cooperation on use of nuclear energy for peaceful purposes and prevention of nuclear proliferation," foreign ministry spokeswoman Jiang Yu said.
But in Japan, the Asahi Shimbun daily called the Vienna vote "a historic mistake" that would weaken global efforts at non-proliferation.
"After North Korea, there are now strong proliferation concerns about Iran," the influential liberal newspaper said.
"This should really be a time to increase the integrity of the NPT and attempt to ease the nuclear crisis. This decision goes completely in the opposite direction," it said.
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