James A. Williamson (February 8, 1829 – September 7, 1902) was a politician and lawyer who served in the Union army during the American Civil War, rising to the rank of brigadier general.

When he was fifteen years old his family moved to Iowa, where he worked as a farmer, before studying law and being admitted to the bar.

In the years prior to the Civil War he served as the chairman of the Iowa State Democratic Committee.

Williamson's division, commanded by Peter J. Osterhaus was temporarily attached to the forces under Joseph Hooker and took part in the Battle of Lookout Mountain.

Briefly during the summer of 1865 he commanded the District of St. Louis and was mustered out of the volunteer service on August 24, 1865, but was on an inspection tour of army posts in the West and did not find out until he returned in October 1865.

In 1864 Williamson had been elected chairman of the Iowa delegation to the National Republican Convention in Baltimore, but did not attend due to duty to the army.