Hopalong Cassidy is a fictional cowboy hero created in 1904 by the author Clarence E. Mulford, who wrote a series of short stories and novels based on the character. Mulford portrayed the character as rude, dangerous, and rough-talking.

He was shot in the leg during a gun fight which caused him to walk with a little "hop", hence the nickname.From the 1930s to the 1950s, the character became indelibly associated with actor William Boyd, who portrayed Cassidy first in a series of sixty-six films from 1935 to 1948, then in children-oriented radio and TV series, both of which lasted until 1952.

Boyd's portrayal of Cassidy had little in common with the literary character, being instead a clean-cut, sarsaparilla-drinking hero who never shot first. The plots of the film, radio and TV series were generally not taken from Mulford's writings.At the peak of the character's popularity in the early 1950s, he spawned enormous amounts of merchandise, as well as a comic strip, additional novels by Louis L'Amour (writing as Tex Burns), and even a short-lived amusement park, "Hoppyland", in Venice, Los Angeles.