Mr Rudd has pledged tough action on the environment
Australian Labor Party leader Kevin Rudd has been sworn in as prime minister, following a landslide victory in parliamentary elections last week.
Immediately after the ceremony, he signed documents to ratify the Kyoto Protocol on climate change, reversing the previous administration's policy.
"This is the first official act of the new Australian government," he said.
Australia's new stance on Kyoto will isolate the US as the only developed nation not to have ratified the treaty.
Mr Rudd is due to attend the UN climate change conference in Bali next week with four of his ministers.
When they heard of Mr Rudd's decision, delegates at the conference erupted in applause.
Mr Rudd's appointment as prime minister ends more than 11 years of conservative government under his predecessor John Howard.
As well as signing up to the Kyoto Protocol, the new government is committed to withdrawing Australia's combat troops from Iraq.
A new approach
Mr Rudd and 29 other ministers took the oath of office at a ceremony in Canberra on Monday morning.
The new line-up has many differences from Mr Howard's administration.
Mr Rudd, a 50-year-old former diplomat, is the first Australian born after the end of World War II to hold the office.
Deputy Prime Minister and Minister for Education and Workplace Relations: Julia Gillard
Treasurer: Wayne Swan
Foreign Minister: Stephen Smith
Environment, Heritage and Arts Minister: Peter Garrett
Climate Change and Water Minister: Penny Wong
Minister of Defence: Joel Fitzgibbon
Attorney-General: Robert McClelland
Deputy Prime Minister Julia Gillard is the first woman to hold the post, and the new government also includes a former rock star, Peter Garrett, as the environment minister.
The minister with responsibility for climate change, Penny Wong, is not only the country's first openly gay minister but the first to be born in Asia.
The new approach to the environment is one of the main ways in which Mr Rudd is signalling a definitive break with the past.
He made the ratification of the Kyoto Protocol one of his key election pledges, and has lost no time in signing the paperwork to bring it into force. Australia will be an official signatory of the Kyoto treaty in 90 days time.
Mr Howard was in power for 11 years
Signing the Kyoto Protocol is "a significant step forward in our country's efforts to fight climate change domestically - and with the international community," Mr Rudd said on Monday.
He said his government would do "everything in its power" to help Australia meet its Kyoto obligations - and has set a long-term target of cutting carbon emissions by 60% of 2000 levels by 2050.
Mr Howard steadfastly refused to sign the Kyoto agreement, arguing that there was no point unless big polluters among developing countries such as China and India were also subject to similar targets.