The Smiley face murder theory (variations include Smiley face murders, Smiley face killings, Smiley face gang, and others) is a theory advanced by retired New York City detectives Kevin Gannon and Anthony Duarte, and Dr. Lee Gilbertson, a criminal justice professor and gang expert at St. Cloud State University.[1] They allege that a number of young men found dead in bodies of water across several Midwestern American states from the late 1990s to the 2010s[2] did not accidentally drown, as concluded by law enforcement agencies, but were victims of a serial killer or killers.

The term "smiley face" became connected to the alleged murders when it was made public that the police had discovered graffiti depicting a smiley face near locations where they think the killer dumped the bodies in at least a dozen of the cases. Gannon wrote a textbook case study on the subject titled "Case Studies in Drowning Forensics".[3][4] The response of law enforcement investigators and other experts has been largely skeptical.