X, sometimes referred to as Mr. X, is a fictional character on the American science fiction television series The X-Files. He serves as an informant, leaking information to FBI Special Agents Fox Mulder and Dana Scully to aid their investigation of paranormal cases, dubbed X-Files. The character serves as a replacement for Deep Throat, who had been killed off in the first season finale, "The Erlenmeyer Flask". X himself would be killed off after appearing in several seasons, eventually being replaced by Marita Covarrubias.

X is portrayed in the series by Steven Williams, and made his debut in the second season episode "The Host", although the character would not appear on-screen until "Sleepless", two episodes later. The role had originally been conceived as female, with Natalija Nogulich cast in the role; however, her initial scenes were deemed unsatisfactory by the producers, leading to her replacement. Williams' portrayal of X was intended to introduce a personality completely different from the character's predecessor, Deep Throat, and was positively received by critics and fans.