John Wayne Gacy (March 17, 1942 – May 10, 1994) was an American serial killer and pederast who raped, tortured, and murdered at least 33 young men and boys. Gacy regularly performed at children's hospitals and charitable events as "Pogo the Clown" or "Patches the Clown", personas he had devised. He became known as the Killer Clown due to his public services as a clown prior to the discovery of his crimes.

Gacy committed all of his murders inside his ranch house near Norridge, a village in Norwood Park Township, metropolitan Chicago, Illinois. Typically, he would lure a victim to his home and dupe him into donning handcuffs on the pretext of demonstrating a magic trick. He would then rape and torture his captive before killing him by either asphyxiation or strangulation with a garrote. Twenty-six victims were buried in the crawl space of his home, and three others were buried elsewhere on his property; four were discarded in the Des Plaines River.

Gacy was convicted of the sodomy of a teenage boy in Waterloo, Iowa, in 1968 and was sentenced to ten years' imprisonment, but served eighteen months. He murdered his first victim in 1972, had murdered twice more by the end of 1975, and murdered at least thirty subsequent victims after his divorce from his second wife in 1976. The investigation into the disappearance of Des Plaines teenager Robert Piest led to Gacy's arrest on December 21, 1978.

His conviction for thirty-three murders (by one individual) then covered the most homicides in United States legal history. Gacy was sentenced to death on March 13, 1980. On death row at Menard Correctional Center, he spent much of his time painting. He was executed by lethal injection at Stateville Correctional Center on May 10, 1994.