William Reibert Mapother Jr.[1] (/ˈmeɪpɒθər/; born April 17, 1965) is an American actor, known for his role as Ethan Rom on the television series Lost and starring in the film In the Bedroom.

Mapother has also had considerable roles in a series of independent films, such as The Lather Effect, Moola, Hurt, and Another Earth.

Mapother starred in The Burrowers as a Native American fighter who joins a posse to help find missing white settlers.

In September 2007, he was elected to a three-year term on the National Board of Directors for the Screen Actors Guild.

He has provided the motion capture work for Agent 47, the main character in the 2012 video game Hitman: Absolution and also provided the voice before series veteran David Bateson was recast.