Russian President Vladimir Putin has signed the Kyoto protocol on climate change - clearing the way for the treaty to come into force next year.
From next year signatories must start cutting emissions
This is Russia's final and crucial stamp of approval for Kyoto.
The pact needed support from countries responsible for 55% of the world's greenhouse gas emissions, which most scientists blame for global warming.
After the United States refused to ratify it, only Russia could enable this threshold to be passed.
Both chambers of Russia's parliament ratified the pact last month, but there are still fears in Moscow that Kyoto could badly affect the country's economic growth.
The BBC's Sarah Rainsford, in Moscow, says Mr Putin agreed to support Kyoto only after European Union leaders offered their support for Russia to join the World Trade Organisation.
More than 100 countries have already signed up to the project, which aims to stem global warming by reducing greenhouse gas emissions.
Early next year - 90 days after Russia formally tells the UN of its ratification - Kyoto signatories must start making cuts that will reduce emissions of six key greenhouse gases to an average of 5.2% below 1990 levels by 2012.
Environmental campaigners and international leaders have welcomed Russia's decision to back the treaty.
Japanese Prime Minister Junichiro Koizumi said: "It's a fresh start for policies to combat global warming. I warmly welcome it."
Russia's agreement was essential because the protocol could only come into force when ratified by developed nations that account for at least 55% of global greenhouse emissions.
After the US pulled out in 2001, saying it would gravely damage the US economy, that figure could only be reached with the support of Russia, which accounts for 17% of world emissions.