Ethan Bennett Farnum (born November 10, 1826 in Cheshire, Massachusetts, date of death unknown) was one of the first residents of Deadwood, South Dakota, who was not a miner or prospector; he was the owner of a general store. Farnum was married to Mary Farnum with three children, Sylvia, age 16, Edward, age 12, and Lyde, age 2 when he arrived in Deadwood. He was the first mayor of the town of Deadwood.

Farnum came to Deadwood from Wisconsin and opened his store in 1876, acquired other Main Street properties, and invested in some of the mining operations such as the Laura Mine and the Prince Oscar Load; he also partnered with other camp entrepreneurs to have the Deadwood to Centennial Toll Road constructed to ease the flow of supplies to their businesses.

On August 18, 1876, he was elected mayor, winning 672 votes out of 1139; in this position he was active in convincing the Dakota Territories to officially recognize the town and establish a nearby Army post, as well as instituting a pest house to quarantine those with communicable diseases (a smallpox epidemic striking the town in 1876) and a system of street cleaning, all to be funded out of licensing fees for town businesses. In December 1876, Farnum established the town's fire department and sent the town's first telegram, to the mayor of Cheyenne, Wyoming. His position also included duties as head of the school board, and Justice of the Peace and judge; he established the town's first school, in November 1876 presided over the town's first marriage, Fannie Garrettson to Daniel Brown, and presided over numerous trials. Since there was no Territorial jurisdiction to his court, there could be no prison sentences, so defendants were either found not guilty and released, or guilty and hanged. Those who were released were often hanged by vigilantes anyway.

Further records are scarce, due to the great fire of September 26, 1879; when these duties became separate positions from the mayorality, Farnum ran for Justice of the Peace but lost. Afterwards, he left for Chicago, Illinois; he is not listed as a town resident in the 1880 census. His mayoral successor is unknown, although Sol Star would eventually be elected to the position in 1884.