Mister Sinister (Nathaniel Essex) is a fictional supervillain appearing in American comic books published by Marvel Comics. Created by writer Chris Claremont, the character was first mentioned as the employer behind the team of assassins known as the Marauders in The Uncanny X-Men #212 (December 1986), and first seen in silhouette in the following issue, #213, which were part of the 1986 "Mutant Massacre" storyline. Mr. Sinister made his first full appearance in Uncanny X-Men #221 (September 1987), his visual appearance designed by artist Marc Silvestri.
Mister Sinister has appeared in a number of X-Men titles including spin-offs and several limited series. The character has also featured in associated Marvel merchandise including animated television series, toys, trading cards, and video games.