Chastain, whose film credits include "The Tree of Life" and "Take Shelter," has been a vegetarian for 15 years and a vegan for five, and even had vegan fried chicken prepared for her on the set of "The Help," PETA said.
"I don't want to torture anything," she said. "[I]t's about trying to live a life where I'm not contributing to the cruelty in the world. … While I am on this planet, I want everyone I meet to know that I am grateful they are here."
Harrelson, who recently starred in "The Hunger Games" and "Rampart," has long maintained a vegan diet.
"I just let people see my energy and how strong I stay," he said.
Previous winners of PETA's Sexiest Vegetarian Celebrities contest include Kristen Wiig, Russell Brand, Olivia Wilde, Carrie Underwood, Prince, Natalie Portman, Shania Twain, Leona Lewis, Andre 3000, Tobey Maguire and Alicia Silverstone.