The actor revealed that he and the Oscar-winning star had a teenage romance after meeting on the set of a 1992 made-for-tv movie.
"[Witherspoon] was my first actress girlfriend," Sisto, 41, said during a Reddit Ama. "We had a long-distance relationship for a long time."
The movie, a drama called Desperate Choices: To Save My Child, was filmed when Witherspoon was around 16 and Sisto was around 17 years old. In the film, Witherspoon played a teenager dying from leukemia.