Pecos Bill (/ˈpeɪkəs/ PAY-kəs) 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 by Tex O'Reilly in the early 20th century and are an example of American "fakelore". Pecos Bill was a late addition to the larger-than-life characters, such as Paul Bunyan or John Henry.

The origin story of the character depicts him as a feral child who was raised by a pack of coyotes. Years later, his long-lost brother convinces Bill that he is not a real coyote. In some tales concerning his final fate, Pecos and his love interest Slue-Foot Sue supposedly end up on the Moon, apparently never to return. In the animated film adaptation Melody Time, Sue alone is stranded on the Moon. A disheartened Bill then leaves civilization to rejoin the coyotes, who start howling at the Moon in honor of Bill's sorrow for Sue.

The first known short story about Pecos Bill, titled The Saga of Pecos Bill, was written by Tex O'Reilly and published in The Century Magazine in October 1923. O'Reilly claimed they were part of an oral tradition of tales told by cowboys during the westward expansion and settlement of the southwest, including Texas, New Mexico, and Arizona. But American folklorists J. Frank Dobie and Richard M. Dorson found that O'Reilly invented the stories as "folklore", and that later writers either borrowed tales from O'Reilly, or added further adventures of their own invention to the cycle.

"Pecos Bill" was also the nickname of Civil War general William Shafter, although this was before O'Reilly created the legend. Shafter was considered a hero in Texas, and even had some legendary poetry written about how tough he was.

According to legend, Pecos Bill is responsible for creating many landmarks. One landmark he is said to have created is the Gulf of Mexico. Apparently, there was a drought in Texas that was so horrible, that Pecos rushed to California and lassoed up a storm cloud and brought it to Texas. It rained so much that the Gulf of Mexico was created. Another story is of him creating the Rio Grande River. He and his horse got stranded in the desert and needed water. So Pecos grabbed a stick and dug the Rio Grande River. One other landmark that he is responsible for is the Painted Desert. He apparently started shooting at a tribe of Indigenous Americans, and as they ran away, the ritual paint they had on them came off and painted the desert.

According to the "legend", Pecos Bill was born in Texas in the 1830s (or 1845 in some versions, the year of Texas's statehood). Pecos Bill's family decided to move out because his town was becoming "too crowded." Pecos Bill was traveling in a covered wagon as an infant when he fell out unnoticed by the rest of his family near the Pecos River (thus his nickname). He was taken in and raised by a pack of coyotes. Years later he was found by his real brother, who managed to convince him he was not a coyote.

He grew up to become a cowboy. Bill used a rattlesnake named Shake as a lasso and another snake as a little whip. His horse, Widow-Maker (also called Lightning), was so named because he was Texas's first and most notorious serial killer, leaving a trail of dead bodies clear across Texas (this is another version of how the Rio Grande was made). Dynamite was said to be his favorite food. It is also said Bill sometimes rode a cougar instead of a horse. On one of his adventures, Pecos Bill managed to lasso a twister. It was also said that he once wrestled the Bear Lake Monster for several days until Bill finally defeated it.

Pecos Bill had a sweetheart named Slue-Foot Sue, who rode a giant catfish down the Rio Grande. He was fishing with the pack when he saw her. Shake, Widow-Maker, and Slue-Foot Sue are as idealized as Pecos Bill.