James R. Barnett, Sr., (May 31, 1842 – January 20, 1917) was an American medical doctor, banker, and Republican politician.

He served two years in the Wisconsin State Assembly, representing northern Winnebago County.

As a young man, he moved to Fond du Lac County, Wisconsin, and taught school there for two years before the outbreak of the American Civil War.

[1] In the summer of 1862, he volunteered for service in the Union Army and was enlisted as a private in Company I of the 1st Wisconsin Cavalry Regiment.

[4] Barnett married Emma G. Scribner shortly after graduating from medical school.