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.

)[6] On February 18, 1878, John Tunstall was killed, allegedly while resisting arrest, by Lincoln County Deputy Sheriffs William Morton, Frank Baker, Jesse Evans, and Tom Hill.

Marshal Robert Widenmann, and a detachment of soldiers disarmed Sheriff Brady's jail guards, put them behind bars, and released the Kid and Brewer.

On April 1, 1878, Regulators Jim French, Frank McNab, John Middleton, Fred Waite, Henry Newton Brown and Billy the Kid ambushed Sheriff William J. Brady and four of his deputies on the main street of Lincoln.

Within hours, however Lt.-Col. Dudley procured arrest warrants for Deputy U.S. Marshall Widenmann and seven other members of the Regulators, who were now rearrested and held at Fort Stanton pending investigation in connection with the murder of Brady.

On May 2, 1878, Widenmann, Alexander McSween, and seven others were incarcerated, this time by Lincoln County Sheriff John Copeland on Dudley's orders, only to be released two days later on lack of evidence.

On June 12, 1878, Rob left Lincoln for the last time, traveling under United States Army escort to Mesilla, New Mexico to testify against Jesse Evans.

[1] According to his daughter Elsie, Widenmann lived in fear of his life for many years because of his role in the Lincoln county war and in bucking such powerful New Mexico politicians as Stephen B.