The King Charles Spaniel is a small dog breed of the spaniel type. In 1903, the Kennel Club combined four separate toy spaniel breeds under this single title. The other varieties merged into this breed were the Blenheim, Ruby and Prince Charles Spaniels, each of which contributed one of the four colours available in the breed.

A spaniel is a type of gun dog. Spaniels were especially bred to flush game out of denser brush. By the late 17th century, spaniels had been specialized into water and land breeds. The extinct English Water Spaniel was used to retrieve water fowl shot down with arrows. Land spaniels were setting spaniels—those that crept forward and pointed their game, allowing hunters to ensnare them with nets, and springing spaniels—those that sprang pheasants and partridges for hunting with falcons, rabbits and smaller mammals such as rats and mice for hunting with greyhounds. During the 17th century, the role of the spaniel dramatically changed as Englishmen began hunting with flintlocks for wing shooting. Charles Goodall and Julia Gasow (1984) write that spaniels were "transformed from untrained, wild beaters, to smooth, polished gun dogs."