Ellis Albert Swearengen, known as Al Swearengen, (b. Oskaloosa, Iowa, July 8, 1845 - d. Colorado, 1904) was a pimp and early entertainment entrepreneur in Deadwood, South Dakota, running the Gem Theater, a notorious brothel, for 22 years, and combining a reputation for brutality with an uncanny instinct for forging political alliances. Swearengen is the basis for a major character of the same name in the HBO television series Deadwood.

Swearengen and his twin brother, Lemuel, were two of ten children of Daniel Swearengen and Keziah (often called Katie) Swearengen of Oskaloosa. Unlike so many of Deadwood's residents who left home at a young age to make their fortunes on the wild frontier, Swearengen remained at home well into his adult years, arriving in Deadwood in the summer of 1876 with his wife, Nettie Swearengen. Nettie would later divorce him on the grounds of spousal abuse, and Swearengen would marry two more times, both marriages ending as the first.

Swearengen was one of the first Deadwood residents not to be a prospector or miner; he represented the beginning of a second wave of residents, attracted there by the promise of riches to be stripped not from the earth, but from the prospectors and miners. He built a small saloon called the Cricket Saloon, which featured as entertainment in its close spaces local miners engaged in what were advertised as "prize fights", although no prizes were actually awarded. Within a year, Swearengen had accumulated enough money to build the much larger and more opulent Gem Variety Theater, which opened on April 7, 1877, featuring the now traditional "prize fights" in addition to stage shows, and, mainly, prostitution.

Swearengen lured desperate young women from far away to Deadwood, then forced them into prostitution through a combination of bullying and physical brutality, by himself and his henchmen. The results were highly lucrative, the Gem bringing in an average of $5,000 a night, sometimes as much as $10,000 (between $90,000 and $180,000 inflation adjusted for 2006). When it burned down along with much of the town on September 26, 1879, Swearengen rebuilt it larger and more opulent than ever, to great public acclaim. Swearengen's talent for canny alliances and financial payoffs kept him insulated from the general drive to clean up the town, including the otherwise successful work of Seth Bullock, until the Gem burned down once again in 1899. He remarried the same year to Odelia Turgeon. It appears that the Methodist church in Deadwood - which also took a moralist crusade to clean up the town - specifically targeted the Gem Theater.

According to his recently rediscovered obituary, Albert Swearengen was found dead in the middle of a suburban Denver street in late 1904. He apparently died of a massive head wound and was not hopping a freight train as is often reported.