Harrelson will co-star with Bruce Willis and Channing Tatum in the film based on the 1968 My Lai Massacre, the Hollywood Reporter said Thursday.
Harrelson will play Col. Henderson, the conflicted officer in charge of the task force that committed the massacre.
About 500 women, children and elderly were slain by U.S. soldiers during the notorious Vietnam War incident.
Willis will portray William R. Peers, the Army general who investigated the massacre and Tatum will play Hugh Thompson Jr., an heroic Army helicopter pilot who aided the villagers and later testified against the soldiers involved.
Production on the film is slated to begin next year.