The Swedish teenager retweeted Trump’s “STOP THE COUNT!” cry on Thursday, while hurling one of his own insults back at him.
“So ridiculous,” Thunberg wrote. “Donald must work on his Anger Management problem, then go to a good old fashioned movie with a friend! Chill Donald, Chill!”
Trump, 74, used the same words to attack the 17-year-old last December, after she was named Time magazine’s Person of the Year.
Trump has long coveted the “Person of the Year” title, and appeared to be upset when he lost the 2019 honour to Thunberg.
He was caught in 2017 with a fake “Person of the Year” cover of himself on the wall at one of his golf clubs.
Time legitimately named him “Person of the Year” in 2016.
Thunberg’s tweet racked up more than 1.6 million likes on Twitter over 24 hours. Trump’s old tweet maxed out at 207,600.
The president ignored Thunberg when the two crossed paths at the United Nations last year, then mocked her passionate speech for climate action on Twitter.
“She seems like a very happy young girl looking forward to a bright and wonderful future,” Trump wrote last September.
In the past, Thunberg has taken the president’s insults and added them to her Twitter biography.
“A teenager working on her anger management problem,” the biography read last December. “Currently chilling and watching a good old-fashioned movie with a friend.”
Trump has repeatedly denied the existence of climate change. He also slashed a wide range of pollution regulations during his tenure as president.
That tenure appeared to be in jeopardy after election day, as mail-ballot totals pushed his opponent, Joe Biden, ahead in the race for the White House.
Trump has baselessly claimed widespread fraud against him, without presenting evidence.
—With files from Reuters