The Hollywood star, who is an environmental campaigner himself, said the 16-year-old’s message should be a “wake-up call to world leaders everywhere that the time for inaction is over”.
Leo posted a picture of the two of them together on Instagram, writing: “There are few times in human history where voices are amplified at such pivotal moments and in such transformational ways – but @GretaThunberg has become a leader of our time.
“History will judge us for what we do today to help guarantee that future generations can enjoy the same livable planet that we have so clearly taken for granted.
“I hope that Greta’s message is a wake-up call to world leaders everywhere that the time for inaction is over.”
The Once Upon A Time In Hollywood actor said he is “optimistic about what the future holds” thanks to Greta and young activists like her.
“It was an honour to spend time with Greta,” he added.
“She and I have made a commitment to support one another, in hopes of securing a brighter future for our planet.”
Leo previously highlighted the issue of climate change during his acceptance speech after winning the Oscar for Best Actor in 2016.
He said: “Climate change is real, it is happening right now. It is the most urgent threat facing our entire species, and we need to work collectively together and stop procrastinating.”
While Greta has the public support of stars like Leo and naturalist David Attenborough, she has proved to be somewhat of a divisive figure.
Both Jeremy Clarkson and Piers Morgan have publicly criticised the young environmentalist, whose activism sparked a global strike movement that has seen millions of youngsters from across the world protest by refusing to go to school.