The climate activist tweeted an image of Mr Trump boarding Marine One for the last time along with the comment: “He seems like a very happy old man looking forward to a bright and wonderful future. So nice to see!”
The mocking comment was a nod to Mr Trump’s own condescending remark about the Swede following her address to the Davos summit in January last year.
At that time, the president tweeted: “She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!”
Mr Trump will not be able to respond in kind this time, however, having been banned permanently from Twitter.
Watch: Twitter Resets @POTUS Account for President Joe Biden
Read more: Trump leaves White House for final time
The pair have had a long-running feud.
In December 2019, Mr Trump said of Ms Thunberg’s recognition as Time magazine’s “person of the year”: "So ridiculous. Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!"
In January last year Ms Thunberg said the 45th US president’s “inaction is fuelling the flames” of climate change, leading him to describe her as “angry”.
He seems like a very happy old man looking forward to a bright and wonderful future. So nice to see! pic.twitter.com/G8gObLhsz9
— Greta Thunberg (@GretaThunberg) January 20, 2021
And in November she reversed Mr Trump’s prior attack by telling him, as he was trying “stop the count” in the US election: "So ridiculous. Donald must work on his Anger Management problem, then go to a good old fashioned movie with a friend! Chill Donald, Chill!"