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.

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.

"Hoppy" and his white horse, Topper, usually traveled through the West with two companions: one young and trouble-prone with a weakness for damsels in distress, the other older, comically awkward and outspoken.

[2] The juvenile lead was successively played by James Ellison, Russell Hayden, George Reeves, Rand Brooks, and Jimmy Rogers.

[6] The series and character were so popular that Hopalong Cassidy was featured on the cover of national magazines such as Look, Life, and Time.

[7] In 1950, more than 100 companies manufactured $70 million of Hopalong Cassidy products,[4] including children's dinnerware, pillows, roller skates, soap, wristwatches (made by Timex), and jackknives.

[15] Mirror Enterprises Syndicate distributed a Hopalong Cassidy comic strip starting in 1949; it was bought out by King Features in 1951, running until 1955.

Fifteen miles east of Wichita, Kansas, at the Prairie Rose Chuckwagon Supper was the Hopalong Cassidy Museum.

The museum and its contents were auctioned on August 24, 2007, owing to the failure of its parent company, Wild West World.

A "Hoppy Museum" consisting of a collection of products endorsed by William Boyd is located at Scott's 10th Street Antique Mall in Cambridge, Ohio.

William Boyd's collection, including Hopalong's TV production materials, is archived at the American Heritage Center at the University of Wyoming.

'"[23] The 1951 song "It's Beginning to Look a Lot Like Christmas" includes a reference to "Hopalong boots" as a holiday gift desired by children.

In 1973, fellow film cowboy Roy Rogers released a nostalgic ballad called "Hoppy, Gene and Me".

In the 1985 film Fletch, the eponymous character, played by Chevy Chase, jokes that he was close to buying a house until he learned that Hopalong Cassidy had killed himself there.

In 2009, the United States Postal Service featured Cassidy as part of a series of stamps depicting early TV characters.