Justus McKinstry (July 6, 1814 – December 11, 1897) was a United States Army officer who served in the Second Seminole War and with merit in the Mexican–American War and in the Third Seminole War. He was appointed a brigadier general and assistant quartermaster in the Union Army in the early days of the American Civil War but his appointment expired without being confirmed by the United States Senate. His actual highest rank was major. He was suspended from his appointment and held under arrest starting November 13, 1861, although his confinement was expanded to the city limits of St. Louis, Missouri after February 22, 1862, in anticipation of a court martial in October 1862. He was convicted of graft, corruption and fraud in the quartermaster's department in the Department of the West. The court recommended his dismissal from the army. On January 28, 1863, after being held in arrest for more than a year, McKinstry was cashiered "for neglect and violation of duty to the prejudice of good order and military discipline." Despite the expiration of his brigadier general appointment without Senate confirmation, some sources, such as Ezra Warner, list McKinstry as a brigadier general. If so regarded, he was one of three Union Army generals who were cashiered. After his dismissal from the Union Army, McKinstry was a speculator and stock broker in New York City, 1864–1867, and land agent in Rolla, Missouri, 1867 – c. 1870, although he spent most of the rest of his life in reduced circumstances in St. Louis.