The US president took to Twitter after Thunberg made an emotional speech at the United Nations Climate Action Summit in New York earlier this week.
'Very happy young girl'
Retweeting a WIRED article which quoted Thunberg talking about the devastation of climate change, Trump wrote: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"
Thunberg responded to Trump's sarcasm by changing her Twitter bio to read: "A very happy young girl looking forward to a bright and wonderful future."
In her emotional speech, which garnered the attention of the world's media, Thunberg accused world leaders of 'stealing her dreams and childhood with their empty words'.
"And yet I'm one of the lucky ones," she added. "People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction, and all you can talk about is money and fairy tales of eternal economic growth. How dare you!
"For more than 30 years, the science has been crystal clear. How dare you continue to look away and come here saying that you're doing enough, when the politics and solutions needed are still nowhere in sight.
"You say you hear us and that you understand the urgency. But no matter how sad and angry I am, I do not want to believe that. Because if you really understood the situation and still kept on failing to act, then you would be evil. And that I refuse to believe."