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 stars as John Nolan on The Rookie and is an executive producer on the show as well as its spin-off series, The Rookie: Feds.

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 theatre. His voice is featured in animation and video games, such as the Bungiegames Halo 3, Halo 3: ODST, Halo: Reach, Destiny, and Destiny 2, along with the 343 Industries game Halo 5: Guardians and the television series M.O.D.O.K. (2021).