The 16-year-old led rally on Friday attracted more than 1,000 people as wildfires burnt across the state, reports aceshowbiz.com.
"The Wolf of Wall Street" actor, a long-time environmentalist with his own foundation, shared two photographs taken with the Swedish green warrior and posted a long message on his Instagram page.
The pair has made a commitment to support each other in their common goal of saving the planet
"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," Leo wrote.
"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".
DiCaprio added: "It is because of Greta, and young activists everywhere that I am optimistic about what the future holds. It was an honor to spend time with Greta. She and I have made a commitment to support one another, in hopes of securing a brighter future for our planet. #FridaysforFuture #ClimateStrike @fridaysforfuture."
The actor's high praise emerged as Thunberg addressed thousands of Los Angeles-area students, teachers and parents at a rally in the city, where she joined young activists focusing their attention on the urgent need to end the production of fossil fuels.
( With inputs from IANS )