Hopalong Cassidy was a popular American radio western program that aired from 1948 to 1952. Based on the movie and television adaptations of the Hopalong Cassidy novels written by Clarence E. Mulford, the show starred William Boyd as Hopalong Cassidy and Andy Clyde as California Carson, his sidekick. The radio show featured new adventures, as well as adaptations from the movies and television episodes.

In the radio show, Cassidy's character was significantly more civilized than in the original novels, acting as a knight of the range and a man of morals. This character transformation made the radio adaptation more appealing as a "juvenile adventure program" and a "western adventure drama."

The radio show gained popularity due to its connection with the existing movie and television series. The radio, television, and movies blended together seamlessly and fed off each other's success, cementing Hopalong Cassidy as a multimedia sensation.