The 27-year-old said the movie, titled Lovelace, would show how the star was used and abused.
"We really wanted to tell her side of the story because it's pretty dark. She was treated terribly because she was a porn star and she was associated with that one movie her whole life.
"It's really nice to be able to validate her in some way. She never got the validation she deserved before she died. I'm glad I did this movie," she said.
Seyfried also stars in the movie adaptation of musical Les Miserables, to be released next month.