"We've made meaningful and unprecedented breakthrough in Copenhagen," the president said.
A senior administration official said the leaders of five key countries, including China and India, agreed to a political "accord" that will "provide the foundation for an eventual legally binding treaty."
Nevertheless, critics are dismissing the agreement as "a sham."
The accord was finalized after Obama "barged into" a meeting between Chinese Premier Wen Jiabao, Indian Prime Minister Manmohan Singh and South African President Jacob Zuma, who were discussing how they would verify any promised cuts in emissions.
"Mr. Premier, are you ready to see me? Are you ready?" Obama said to Wen, according to a pool reporter who was with the president.
The leaders proceeded to talk for roughly 45 minutes and emerged with a non-binding political accord, but Obama was unable to get the four leaders to commit to a deadline of 2010 for a legally binding international climate change treaty.
Obama said the nations will submit to international consultation and analysis of their efforts to cut emissions, similar to how the World Trade Organization examines economic progress.
It won't be legally binding and there is no set timeframe, but it will allow each country to show to the world its progress, and countries will lay out the targets in an appendix to the agreement.
The president said he hopes to make the agreement legally binding in a year, but he said it will be "very hard and take some time."
"We were modest in what we thought we could accomplish ... but believe whatever we promise we can deliver on," the president said following the meeting with the three leaders.
Environmental groups were disappointed that a more concrete agreeement did not emerge from the conference.
Friends of the Earth, in a statement, called the accord, "a sham agreement with no real requirements for any countries. This is not a strong deal or a just one -- it isn't even a real one. It's just repackaging old positions and pretending they're new."
The Chinese had for weeks been reluctant to agree to ways for the international community to verify that they're abiding by pledges to reduce greenhouse gas emissions. But today, Chinese officials agreed to report to an international monitoring mechanism, officials said.
"For the first time these emerging economies have agreed to take significant action to combat climate change," the official said, adding that the leaders of the four nations have agreed to the core components Obama laid out in his address this morning upon arriving in Denmark -- mitigation, transparency, and financing.
He defined "mitigation" as every major economy putting forward "decisive national actions that will reduce their emissions, and begin to turn the corner on climate change." The U.S. goal is to reduce greenhouse gas emissions "in the range of 17 percent by 2020, and by more than 80 percent by 2050 in line with final legislation."