Blue-eyed DiCaprio was spotted listening earnestly as the Argentine pontiff explained that he was presenting the actor with a copy of his hard-hitting thesis on climate change, Laudato Si, published last year.
The 41-year old star won the Crystal Award at the World Economic Forum in Davos this month for his green credentials.
The movie great, who rose to fame with "Titanic" and has been nominated for a best actor Oscar for his role in "The Revenant", set up a foundation in 1998 dedicated to protecting endangered wildlife and vulnerable eco-systems.
Raised in Los Angeles in a Catholic family of Italian and German origins, atheist DiCaprio - looking dapper in a black suit and tie - was all smiles with the white-robed pope, who has repeatedly blamed mankind for climate change.
He may also be hoping the photo-op with the popular pontiff will give his Oscar nomination a boost: the actor has been nominated four times but never won one of the coveted gold statuettes.