Relentless is a 1989 American crime thriller film directed by William Lustig and starring Judd Nelson, Robert Loggia and Leo Rossi.

Relentless was the first in a series of four films starring Leo Rossi as detective Sam Dietz trying to stop a serial killer.

Sam Dietz is a rookie Los Angeles detective recently transferred from New York City.

The killer is Arthur "Buck" Taylor, the son of a former LAPD cop whose motive for killing is frustration over not having been accepted to the force and failure in the eyes of his father.

After the success of Maniac Cop, producer Leonard Shapiro of Shapiro-Glickenhaus Entertainment wanted to do another picture with director William Lustig, and presented him with a bunch of scripts that had been submitted to the company.