The star made the confession as she was being interviewed in the green room just before the start of the 83rd Academy Awards.
She said she supported Team Edward (for vampire Edward Cullen) over Team Jacob (for werewolf Jacob Black) in the hugely popular Twilight books and movies.
Witherspoon recently made Water For Elephants with Robert Pattison, who plays Cullen.
Asked if everyone in her family now wore Team Edward T-shirts, she laughed, then added: “I guess I have to be Team Edward now as I know Rob so well!”
Witherspoon also spoke about Water For Elephants, which is due for release later this year.
She said: “I’m really excited about the movie. It stars Robert Pattinson and Christoph Waltz. It’s a beautiful story about a 1930s circus and it’s a love story. It’s really nice. I saw it last week and it’s great.”