Robert Jenkins Onderdonk (January 16, 1852[2] – July 2, 1917) was an American painter and art teacher, born in Catonsville, Maryland.

He assisted in organizing "The Brass Mug Club," a group of San Antonio artists that would go out into the Texas Hill Country to paint.

The club included José Arpa, Leo Cotton, Rolla Taylor, Tom Brown and Ernst Raba.

Onderdonk's masterpiece, the 1903 painting Fall of the Alamo, was displayed at the 1904 Louisiana Purchase Exposition in St. Louis, and is now held in the Texas State Archives.

Occupying central position in the painting is Davy Crockett, a depiction reproduced countless times in print; Crockett is portrayed in iconic style, "swinging his flintlock over his head to club the Mexicans advancing through a hanging cloud of gun smoke.