China, the EU and India, which along with the US make up the four biggest emitters of carbon dioxide, have restated their commitment to the 2015 Paris Agreement on climate change.
Top EU official Donald Tusk said at a summit with China that Brussels and Beijing would step up co-operation.
Mr Trump announced the US was leaving for economic reasons, saying the deal would cost American jobs.
Secretary of State Rex Tillerson said the US would still curb emissions.
Amid widespread international condemnation of the US decision, Russian President Vladimir Putin said he would “not judge” Mr Trump.
The Paris agreement commits the US and 194 other countries to keeping rising global temperatures “well below” 2C above pre-industrial levels and “endeavour to limit” them even more, to 1.5C.
The UN World Meteorological Organisation said on Friday that, in the worst scenario, the US pullout could add 0.3C to global temperatures by the end of the century.
What do the EU and China say?
European Council President Donald Tusk said after meeting Chinese Premier Li Keqiang that the two powers took their responsibilities seriously.
“Today, China and Europe have demonstrated solidarity with future generations and responsibility for the whole planet,” he told reporters at a joint news conference.
“We are convinced that yesterday’s decision by the United States to leave the Paris agreement is a big mistake, bigger than not ratifying the Kyoto Protocol, because Paris is fairer.
“But the fight against climate change and all the research, innovation and technological progress it will bring will continue with or without the United States.”
Mr Tusk was echoing similar pledges by EU politicians such as German Chancellor Angela Merkel who said the path set out by Paris was “irreversible”.
A spokeswoman for the Chinese foreign ministry, Hua Chunying, said China was ready to take a leading role in the fight against climate change.
“In the future, China will continue to tackle climate change in all ways, will proactively participate in the multilateral process of tackling climate change and resolutely uphold the global climate management process,” she said.
Indian Environment Minister Harsh Vardhan said: “As far as the Paris accord is concerned… our government is committed, irrespective of the stand of anyone, anywhere in the world.”
Mr Trump characterised the Paris agreement as a deal that aimed to hobble, disadvantage and impoverish the US.
He said it would cost the US $3tn (£2.3tn) in lost GDP and 6.5 million jobs – while rival economies like China and India were treated more favourably.
Mr Trump said he was fulfilling his “solemn duty to protect America and its citizens”.
Secretary of State Rex Tillerson said the US had a “terrific record on reducing our own greenhouse gas emissions”.