Trump will announce his verdict on America's participation in the 196-party accord during a keenly awaited televised address from the White House Rose Garden at 1900 GMT.
Ever the showman, the 70-year-old gave his decision a reality-TV-style tease, refusing to indicate his preference either way and coyly telling reporters "you're gonna find out very soon."
Leaks from inside the White House suggested Trump has decided to pull out of the 2015 Paris Agreement, a move that would seriously hamper efforts to cut emissions and limit global temperature increases.
The United States is the world's second largest emitter of greenhouse gases, after China.
But White House officials also voiced caution, saying the famously unpredictable president could yet decide to delay departure by years or merely review America's domestic emissions targets.
Meeting those targets largely depends on the actions of US states and businesses, rather than the federal government in Washington.
Opponents of withdrawal -- said to include Trump's own daughter Ivanka -- have warned that America's reputation and its leadership role on the world stage are also at stake, as well as the environment.
Nicaragua and Syria are the only countries not party to the Paris accord, the former seeing it as not ambitious enough and the latter being racked by a brutal civil war.
Diplomatic pressure
Trump's long wind-up has prompted fierce lobbying, with his environmental protection chief Scott Pruitt and chief strategist Steve Bannon urging the president to leave.
"I'm hearing from a lot of people, both ways. Both ways," Trump said on the eve of his statement.
A dozen large companies including oil major BP, agrochemical giant DuPont, Google, Intel and Microsoft, have also urged Trump to remain part of the deal.
Tesla and SpaceX boss Elon Musk said he would have "no choice" but to leave White House-backed business councils if Trump pulls out.
On the diplomatic front, German Chancellor Angela Merkel led the remain camp, publicly describing the deal as "essential," and suggesting other countries would press ahead regardless.
Trump raised alarm bells when he refused to sign up to a pledge on the deal at last week's G7 meeting in Italy.
Merkel on Saturday labelled the result of the "six against one" discussion "very difficult, not to say very unsatisfactory".
European Commission President Jean-Claude Juncker was less diplomatic, all-but accusing the US president of being ill informed.
"I am a transatlanticist. But if the US president in the next hours and days says that he will get out of the Paris accord, then it's the responsibility of Europe to say: that's not acceptable."
He noted that it would take three or four years to exit from the Paris deal, and revealed that world leaders had sought to explain that to Trump at the G7 summit.
"As it appears, that attempt failed," said Juncker. "But a rule is a rule. And everyone must respect it. Not everything that is a rule, and not everything that stands in an international agreement is fake news."
China pledge
Hours ahead of Trump's announcement, China's Premier Li Keqiang also pledged to stay the course on implementing the climate accord, and urged other countries to do likewise.
"China will continue to implement promises made in the Paris Agreement, to move towards the 2030 goal step by step steadfastly," Li said in a Berlin joint press conference with Merkel.
"But of course, we also hope to do this in cooperation with others."
China has been investing billions in clean energy infrastructure, as its leaders battle to clear up the choking pollution enveloping its biggest cities, including Beijing.
China and the US are responsible for some 40 percent of the world's emissions and experts warn is vital for both to remain in the Paris agreement if it is to succeed.
The leader of Asia's other behemoth, Indian Prime Minister Narendra Modi, said this week failing to act on climate change would be "morally criminal".
Contradictory signals
A US withdrawal would come less than 18 months after the historic pact was adopted in the French capital, the fruit of a hard-fought agreement between Beijing and Washington under Barack Obama's leadership.
The Paris Agreement commits signatories to efforts to reduce greenhouse gas emissions that cause global warming, which is blamed for melting ice caps and glaciers, rising sea levels and more violent weather events.
They vowed steps to keep the worldwide rise in temperatures "well below" two degrees Celsius (3.6 degrees Fahrenheit) from pre-industrial times and to "pursue efforts" to hold the increase under 1.5 degrees Celsius.
Since taking office on January 20, Trump, who has called climate change a "hoax", has sent contradictory signals on the Paris deal.
When asked on Tuesday whether Trump believes human activity is contributing to climate change, White House spokesman Sean Spicer told reporters, "Honestly, I haven't asked him that. I can get back to you."
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