John Wayne Gacy (March 17, 1942 – May 10, 1994) was an American serial killer and sex offender known as the Killer Clown who assaulted 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 was also active in his local community as a Democratic Party precinct captain and building contractor.

According to Gacy, all of his murders were committed inside his ranch house near Norridge, a village in Norwood Park Township, metropolitan Chicago, Illinois. Typically, he would lure a victim to his home, dupe him into donning handcuffs on the pretext of demonstrating a magic trick, 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 10 years' imprisonment, but served 18 months. He murdered his first victim in 1972, had murdered twice more by the end of 1975, and at least 30 subsequent victims were killed 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 33 murders was the most by one individual in United States history at the time. 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.