The 72-year-old Terminator star posted a series of photos with the 16-year-old as they rode their bikes together in Santa Monica, California. They joined the actor's daughter Christina and Thunberg's father Svante.
"It was great to see my friend and one of my @GretaThunberg heroes last week and bike ride together for Santa Monica, and I was very happy to introduce her to my daughter Christina. Keep breathing in, Greta!" Schwarzenegger wrote on social networks.
On Friday, Thunberg met with actor and environmentalist Leonardo DiCaprio In Los Angeles.
In an Instagram post, he called her "the leader of our time".
"There are few times in human history when voices are amplified at such crucial times and in transformational ways – but Greta Thunberg has become the leader of our time," he wrote.
“History will judge us by what we do today to help ensure that future generations can enjoy the same habitable planet we so clearly take for granted,” he wrote, adding: “I hope Greta's message is a warning to world leaders in everywhere the time of inaction has ended. "
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 are committed to supporting each other, hoping to secure a better future for our planet," he added.
The Swedish teenager made headlines when she sailed from England to New York earlier this year. Her future Friday campaign has gone global in recent months and she made headlines for scolding world leaders at a United Nations climate conference in September.
Thunberg also made appearances on "The Ellen DeGeneres Show" and "The Daily Show with Trevor Noah".
Lucia Suarez of Fox News contributed to this report.