Trump, who has insulted 16-year-old Thunberg before by sarcastically describing her as a 'very happy girl', tweeted about Thunberg to his 67.5 million followers.
"So ridiculous. Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!" Trump wrote.
Thunberg has since changed her Twitter bio to: "A teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend."
'Ignore the doubters'
Former First Lady Michelle Obama was one of many to show support to Thunberg for her work against the climate crisis.
"Greta Thunberg, don't let anyone dim your light," Obama tweeted. "Like the girls I've met in Vietnam and all over the world, you have so much to offer us. Ignore the doubters and know that millions of people are cheering you on."
Another user added: "Greta Thunberg is a child. A girl. Someone with Asperger syndrome. And she's an inspiration. You're the United States president. And you inspire no one. And you're a scourge on the world."