Boleslav William Felix Robert Sienkiewicz is an American artist known for his work in comic books—particularly for Marvel Comics' New Mutants, Moon Knight, and Elektra: Assassin. He is the co-creator of the character David Haller / Legion, the basis for the FX television series Legion.

Elektra: Assassin is an eight-issue limited series published by Epic Comics, an imprint of American company Marvel Comics, between August 1986 and March 1987. Written by Frank Miller and illustrated by Bill Sienkiewicz, Elektra: Assassin satirizes ultra-violence, politics, comic book clichés like ninjas and cyborgs, and the portrayal of women.