The former governor of California attended the pontiff's General Audience on Wednesday, a weekly meeting when the pope welcomes pilgrims and visitors.
Writing on Twitter, Schwarzenegger said: "It was my great honor to meet His Holiness. I am a huge fan - a true leader for the Church & a steward for all of God's creatures."
The Austrian-born politician, who first shot to prominence as a bodybuilder and actor, came to the Vatican in a bid to raise awareness of R20 - Regions of Climate Action, a non-governmental organisation he founded in 2010.
The group works with local authorities around the world to develop and promote carbon-efficient and climate resilient economic development projects.
During his term as the governor of California between 2003 and 2011, Schwarzenegger clamped down of vehicle emission standards in the state.
After what state-run Vatican Radio described as a "brief greeting" from the pope, Schwarzenegger showed the Catholic leader some of R20's work.
The Terminator film franchise star later said of the pope: "His environmental work inspires me and I look forward to working together for a clean energy future."