Leavenworth (pop. 33,656), city in northeastern Kansas, on the Missouri River. The oldest city in the state, it was organized in 1854 by proslavery settlers near Ft. Leavenworth on the Santa Fe Trail. In 1855 it became the first incorporated town in Kansas Territory. Leavenworth is an industrial and marketing center producing wood, meat, and dairy products, structural steel, greeting cards, and mill machinery. It is the site of Leavenworth Prison.