Nathan Christopher Fillion (; born March 27, 1971) is a Canadian-American actor and voice actor best known for the lead role of Captain Malcolm Reynolds in the television series Firefly and its feature film continuation, Serenity, as well as his role as Richard Castle on the ABC series Castle.

He has acted in traditionally distributed films like Slither and Trucker, Internet-distributed films like Dr. Horrible's Sing-Along Blog, television soap operas and sitcoms, and theater. Fillion has also voiced several video games, including the Bungie titles Halo 3, Halo 3: ODST, Halo: Reach, Destiny, and Destiny 2, along with the 343 Industries video game Halo 5: Guardians.

Fillion first gained recognition for his work on One Life to Live in the contract role of Joey Buchanan, for which he was nominated for Daytime Emmy Award for Outstanding Younger Actor in a Drama Series, and for his supporting role as Johnny Donnelly in the sitcom Two Guys and a Girl.