Pecos Bill is a fictional cowboy and folk hero in stories set during American westward expansion into the Southwest of Texas, New Mexico, Southern California, and Arizona. These narratives were invented as short stories in a book by Tex O'Reilly in the early 20th century and are an example of American folklore. Pecos Bill was a late addition to the "big man" idea of characters, such as Paul Bunyan or John Henry.

Edward Sinnott "Tex" O'Reilly was an American soldier of fortune, writer, journalist, and film actor. He is said to have fought in ten wars under many flags. Initially serving in the U.S. Army in the Spanish–American War, the Philippine–American War, and the Boxer Rebellion, he would claim to fight in several conflicts in Central America and to have fought with Pancho Villa in Mexico and claimed to have fought in the Rif War with the Spanish Foreign Legion in North Africa. He worked as a reporter for the Associated Press. He wrote an autobiography, Roving and Fighting, and Lowell Thomas wrote Born to Raise Hell about him. The latter book has been reprinted and is distributed by The Long Riders' Guild Press. He was the author of Pecos Bill.