Donald Trump described fears over climate change as a "scam".
The former US president said the prime minister was “smart” for watering down a string of green policies.
Sunak announced last week that he was delaying the ban on the sale of new petrol and diesel cars from 2030 to 2035, while the transition from gas boilers to heat pumps in homes will also be postponed.
The PM insisted the UK would still achieve net zero by 2050, but in a way which does not impose unnecessary costs on consumers.
His announcement sparked a furious backlash among senior Tories, but it has found favour with Trump.
Writing on his own social media platform Truth Social, he said: “Prime minister Sunak of the United Kingdom has very substantially rolled back the ridiculous “Climate Mandates” that the United States is pushing on everyone, especially itself.
“I always knew Sunak was smart, that he wasn’t going to destroy and bankrupt his nation for fake climate alarmists that don’t have a clue.”
He added: “Congratulations to Prime Minister Sunak for recognising this SCAM before it was too late!”
Responding to Trump’s comments, the PM’s spokesman said: “The prime minister’s view is that the changes he set out last week are about the fairest, credible path to net zero.
“The UK was and continues to be the country with the most rigorous and ambitious commitments around climate change.”