Nathan Fillion (born March 27, 1971) is a Canadian-American actor. He played the leading roles of Captain Malcolm "Mal" Reynolds on Firefly and its film continuation Serenity, and Richard Castle on Castle. As of 2018, he was starring as John Nolan on The Rookie.

Fillion has acted in traditionally distributed films like Slither and Trucker, Internet-distributed films like Dr. Horrible's Sing-Along Blog, television soap operas, sitcoms and theater. His voice is also featured in animation and video games, such as Hal Jordan / Green Lantern in various DC Comics projects, the Bungie games Halo 3, Halo 3: ODST, Halo: Reach, Destiny and Destiny 2, along with the 343 Industries game Halo 5: Guardians, and Wonder Man in Marvel's M.O.D.O.K. (2021).

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 the Daytime Emmy Award for Outstanding Younger Actor in a Drama Series, as well as for his supporting role as Johnny Donnelly in the sitcom Two Guys and a Girl.