The actress has signed up to join the case of The Young Pope - an eight-part series based on the Church.
Keaton, who won the best actress Oscar in 1977 for Annie Hall, will play Sister Mary in the drama series.
It's directed by Italian filmmaker Paolo Sorrentino and will tell the story of the fictional, American-born Pope Pius XIII, played by Law.
Sorrentino described the show, a co-production by Sky, HBO and Canal +, as a story of "faith" and how people "handle and manipulate power".
Keaton shot to fame in The Godfather, playing the wife of Al Pacino's mafia boss, and her career includes hits such as The First Wives Club and Marvin's Room.