"Don't let anyone dim your light. Like the girls I've met in Vietnam and all over the world, you have so much to offer us all," Obama tweeted early Friday morning while she was on a trip to the country with the Obama Foundation and Girls Opportunity Alliance. "Ignore the doubters and know that millions of people are cheering you on."
On Thursday morning, Trump, 73, wrote "so ridiculous" in response to a tweet from actress and producer Roma Downey congratulating Thunberg on being named Time magazine's 2019 person of the year.
"Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend!" the president continued. "Chill Greta, Chill!"
Seemingly in response to his tweet, Thunberg changed her Twitter bio to read: "A teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend."
Later Thursday, a verified Twitter account managed by the Trump 2020 campaign tweeted a doctored cover of Time's 2019 Person of the Year cover -- with Trump's head photoshopped onto Thunberg's body.
All the tweets come after Time magazine's announcement of Thunberg as Person of the Year, which represents the person or group, idea or object, that "most affected the news and our lives, for good or ill, and embodied what was important about the year, for better or for worse."
Time Editor-in-Chief Edward Felsenthal said while making the announcement Wednesday that Thunberg "represents a broader generational shift in the culture that we're seeing from the campuses of Hong Kong to the protests in Chile to Parkland, Florida, where the students marched against gun violence, where young people are demanding change urgently."
The president was a finalist for the title this year and has demonstrated a persistent interest in who gets the titled each year. He was selected as Person of the Year in 2016.
This is not the first time the president has gone after Thunberg on Twitter -- and not the first time she's acknowledged it on the platform.
"She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!" Trump tweeted in September after Thunberg gave a fiery speech at the United Nations General Assembly criticizing world leaders for inaction on climate change
Thunberg responded then by changing her Twitter bio to read: "A very young happy girl looking forward to a bright and wonderful future."