Kelly pool (also known as pea pool, pill pool, keeley, the keilley game, and killy) is a pool game played on a standard pool table using a standard set of 16 pool balls. Gameplay involves players each drawing one of 15 numbered markers called peas or pills at random from a shake bottle, which assigns to them the correspondingly numbered pool ball, kept secret from their opponents, but which they must pocket to win the game. Kelly pool is a rotation game, which means that players must contact the lowest numbered object ball on each shot first until the opportunity to pocket their own is presented. Two rule variants are set forth under rules promulgated by the Billiard Congress of America (BCA). In the simpler form, the object of play starts and ends with the goal of pocketing one's secret ball. In the second, in addition to the goal of pocketing one's secret ball, points are scored in various ways. In the instance where pills are unavailable, a cloth may be used to cover the balls, which are then chosen blindly, recorded, and replaced for play.

Reportedly invented by Chicagoan Calistus "Kelly" Mulvaney in 1893, kelly pool was a popular game during the early to mid-20th century. Mentions of it were at one time common in US newspapers, often painting it in a negative light, as its play was considered a stronghold of gambling. Authorities in various parts of the United States at times called for a moratorium on the game's play. Until 1964, in fact, playing the game was a fineable offense in Montana.

Many billiard-specific and etymological sources point to kelly pool, or an early version of the game called kelly rotation, as the origin of the common idiom, "behind the eight-ball". Some publications assume the expression to be eponymously derived from the game of eight-ball, but the expression came into use before eight-ball was popularized. The predecessor to the BCA, the National Billiard Association, meanwhile, holds that the expression simply emanates from the fact that the 8 ball, being black-colored, is harder to see than other balls, thus resulting in an association with any difficult position.