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. 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 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." The response of law enforcement investigators and other experts has bee