The Hollywood star said the pair has made a commitment to support one another in their fight for climate action.
Two of the world's biggest voices in the fight for climate action have joined forces, sending fans into meltdown.
Hollywood A-lister Leonard DiCaprio met Swedish teen activist Greta Thunberg in California, capturing the moment on DiCaprio's Instagram.
He posted that the pair have joined forces "in hopes of securing a brighter future for our planet".
DiCaprio, who recently pledged $7 million to preserve the Amazon rainforest yet is widely criticised for his use of private jets, said he was honoured to meet the young climate activist.
"There are few times in human history where voices are amplified at such pivotal moments and in such transformational ways but Greta Thunberg has become a leader of our time," he wrote on Instagram.
"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."
Since her solo school-day strike outside the Swedish parliament in 2018 went viral, Ms Thunberg’s activism has taken on a life of its own.
In September, she rattled world leaders with her emotional speech at the UN General Assembly, in which she told them they had "betrayed" young people through their inertia over the climate crisis.
DiCaprio said he hoped the 16-year-old's message was a wake-up call to leaders everywhere that "the time for inaction is over".
"It is because of Greta, and young activists everywhere that I am optimistic about what the future holds," he wrote on Instagram.