Trump, 73, gave a special address at the conference of world and business leaders at 11 a.m. local time on Tuesday. Thunberg, 17, spoke shortly after.
Conference organizers say climate change is a key theme of this year's event.
Before giving his remarks, President Trump told reporters "I'm a big believer in the environment, the environment is very important to me." However, during his speech, he focused primarily on economic gains and called on attendees to "reject the perennial prophets of gloom and doom and their predictions of the apocalypse." Thunberg gave the opening remarks at a panel on "avoiding the climate apocalypse."
Trump went on to say that the "alarmists" warning of climate catastrophe demand "absolute power to dominate, transform, and control every aspect of our lives."
While Thunberg has become a symbol of the youth-led movement, calling for urgent action on climate change, the Trump administration has taken steps to show it does not consider it a priority, including withdrawing the U.S. from the Paris Climate Agreement. Trump has previously called climate change a hoax.
During her speech at the World Economic Forum, Thunberg called attention to the Trump administration's policy, saying "the fact that the U.S.A. is leaving the Paris accord seems to outrage and worry everyone, and it should." But she continued to say that attendees should also be concerned with other countries' failures to meet the standards set by the agreement.
The last time the two were in the same room, at the United Nations Climate Action Summit in September, a photo emerged on social media showing them crossing paths. It quickly went viral.
WEF founder Klaus Schwab told The Associated Press that both Trump and Thunberg would be welcomed at Davos.
"I think both voices are necessary," he said. "The environment will play a particularly important role during this meeting."
Trump announced during his speech that the U.S. would join the "One Trillion Trees Initiative" launched at the World Economic Forum, an initiative aiming to offset carbon emissions.
In her speech, Thunberg said "Planting trees is good of course, but it’s nowhere near enough of what needs to be done," adding "We don’t need a “low carbon economy.” We don’t need to “lower emissions.” Our emissions have to stop." She also called on world leaders to immediately "immediately and completely divest from fossil fuels."
Thunberg made headlines late last month when she said she "wouldn't have wasted my time" speaking to Trump about climate change.
"I don't think I would have said anything because he's obviously not listening to scientists and experts, why would he listen to me?" she told the BBC's Today radio program. "I probably wouldn't have said anything, wouldn't have wasted my time."
The teen added that she thinks Trump may see the growing climate movement among young people as a "threat."
“We demand that at this year’s forum, participants from all companies, banks, institutions and governments immediately halt all investments in fossil fuel exploration and extraction, immediately end all fossil fuel subsidies and immediately and completely divest from fossil fuels,” she wrote.
The president has frequently attacked the teen climate activist on social media, tweeting that it is "so ridiculous" that Thunberg was selected as Time magazine's 2019 person of the year and telling her to "chill."
"Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend!" the president wrote. "Chill Greta, Chill!"
Thunberg seemed to take it in stride. Shortly after, 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."