The "Terminator" star, 72, posted a series of photos with the 16-year-old as they went for a bike ride together around Santa Monica, Calif. They were joined by the actor's daughter, Christina, and Thunberg's father, Svante.
"It was fantastic to see my friend and one of my heroes @GretaThunberg last week and go on a bike ride around Santa Monica together and I was so pumped to introduce her to my daughter Christina. Keep inspiring, Greta!" Schwarzenegger wrote on social media.
On Friday, Thunberg met up with actor and environmentalist Leonardo DiCaprio in Los Angeles.
In an Instagram post, he called her a “leader of our time.”
“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.
“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,” he wrote, adding: “I hope that Greta’s message is a wake-up call to world leaders everywhere that the time for inaction is over.”
DiCaprio said it was an “honor” to meet with Thunberg and that he is “optimistic about what the future holds” thanks to young activists like her.
“She and I have made a commitment to support one another, in hopes of securing a brighter future for our planet,” he added.
The Swedish teenager made headlines when she traveled by sailboat from England to New York earlier this year. Her Fridays for Future campaign has gone global in recent months and she made headlines for scolding world leaders at a United Nations climate conference in September.
Thunberg has also made appearances on "The Ellen DeGeneres Show" and "The Daily Show with Trevor Noah."
Fox News' Lucia Suarez contributed to this report.