"Treehouse of Horror XXVII" is the fourth episode of the twenty-eighth season of the American animated television series The Simpsons, the 27th episode in the Treehouse of Horror series of Halloween specials, and the 600th episode of the series overall. The episode was directed by Steven Dean Moore and written by Joel H. Cohen. It aired in the United States on Fox on October 16, 2016.

Robert Underdunk Terwilliger Jr., PhD, better known as Sideshow Bob, is a recurring character in the animated television series The Simpsons. He is voiced by Kelsey Grammer and first appeared in the episode "The Telltale Head". Bob is a self-proclaimed genius who is a graduate of Yale University and a champion of high culture, including the adoption of a transatlantic accent, similar to that of Grammer's portrayal of Dr. Frasier Crane from the sitcoms Cheers and Frasier. He began his career as a sidekick on Krusty the Clown's television show, but after enduring constant abuse, Bob framed his employer for armed robbery in "Krusty Gets Busted", only to be foiled by Bart Simpson, and sent to prison. Bob started seeking revenge against Bart while in prison, and the two became feuding arch-enemies.