The Once Upon A Time In Hollywood star, who described meeting Greta as an 'honor', said the pair have 'committed to support one another' in the hopes of 'securing a brighter future for our planet'.
'History will judge us'
"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," Dicaprio wrote to his 37.1 million Instagram followers.
"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. It is because of Greta, and young activists everywhere that I am optimistic about what the future holds.
Recently, DiCaprio's environmental organization Earth Alliance pledged $5 million to help tackle the Amazon forest fires.
According to a post on Instagram, the money will be used to 'focus critical resources for indigenous communities and other local partners working to protect the life-sustaining biodiversity of the Amazon'.
"100 percent of your donation will go to partners who are working on the ground to protect the Amazon," the post added.
"Earth Alliance is committed to helping protect the natural world. We are deeply concerned about the ongoing crisis in the Amazon, which highlights the delicate balance of climate, biodiversity, and the wellbeing of indigenous peoples."