Leavenworth is the largest city and county seat of Leavenworth County, in the U.S. state of Kansas and is a suburb of Kansas City, Missouri. Located in the northeast portion of the state, it is situated on the west bank of the Missouri River. The population was 35,420 at the 2000 census. Leavenworth, founded in 1854, was the first incorporated city in Kansas.

It sprang up south of Fort Leavenworth, the oldest active Army post west of the Mississippi, which was established in 1827 by Colonel Henry Leavenworth. Leavenworth became the first city in Kansas when it was founded in 1854. Leavenworth's main industry is prisons. It is the site of a large federal prison and several smaller prisons, including the military's primary prison, the United States Disciplinary Barracks. Many of the residents have nicknamed their city "Heavenworth".

Leavenworth is a conservative community with a diverse population. It is home to University of Saint Mary (Kansas), operated by the Sisters of Charity.

Leavenworth is a sister city to the New South Wales city of Wagga Wagga in Australia.