US President Barack Obama said the move was “historic”, as he set a new goal of reducing US levels between 26%-28% by 2025, compared with 2005 levels.
China did not set a specific target, but said emissions would peak by 2030.
The two countries also agreed to reduce the possibility of military accidents in the air and sea.
The news came during a state visit by Mr Obama to Beijing, which followed a major Asian regional summit.
It is the first time China, the world’s biggest polluter, has set an approximate date for emissions to peak.
The two countries together produce about 45% of the world’s carbon dioxide.
The unexpected announcement is a bid to boost efforts to secure a global deal on reducing emissions after 2020, to be finalised next year in Paris.
“We agreed to make sure that international climate change negotiations will reach an agreement in Paris,” said Mr Xi, speaking to reporters after the announcement.
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