The actor shared a picture of himself with the climate change activist on Instagram.
He wrote he hoped “Greta’s message is a wake-up call to world leaders everywhere” and that the “time for inaction is over.”
He also praised the teenager for her work and said because of her he is “optimistic about what the future holds”.
he image has had almost 4 million likes since it was posted 22 hours ago.
Greta Thunberg has helped start an international youth movement against climate change.
The Swedish teenager first staged a “School Strike for Climate” in front of the Swedish Parliament in August last year.
Her strike has inspired students from around the world, leading tens of thousands of students from Germany, Japan, the UK, Australia and many more to join her #FridaysforFuture demonstrations.
Leo wrote: “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 liveable planet that we have so clearly taken for granted.”