Robert A. Widenmann (January 24, 1852 – April 13, 1930) was a Deputy United States Marshal and associate of Billy the Kid during the Lincoln County War.
Widenmann was born January 24, 1852, to German-born parents in Ann Arbor, Michigan. Widenmann’s father was the Bavarian Consul in Ann Arbor. As a teen-ager, Widenmann was sent back to the family’s native Germany for his schooling. He lived there for several years with a grandfather.
Returning to America in 1871, Widenmann settled in New York City, where he remained for two years before drifitng west, finally arriving in Lincoln in mid-February 1877. In New Mexico, he would be appointed Deputy Marshal by U.S. Marshal for New Mexico Territory John Sherman Jr.