Donaldsonville (historically French: Lafourche-des-Chitimachas) is a city in and the parish seat of Ascension Parish, Louisiana, United States, along the west bank of the Mississippi River. The population was 7,605 at the 2000 census. It is part of the Baton Rouge Metropolitan Statistical Area.
Acadians began to settle in the area in 1765 and Spanish Isleños also settled here. In 1772 when the territory was under Spanish rule, the militia constructed La Iglesia de la Ascensión de Nuestro Señor Jesucristo de Lafourche de los Chetimaches (the Ascension of Our Lord Catholic Church of Lafourche of the Chitimaches) to serve the area. The region returned later to French control and then was part of the Louisiana Purchase in 1803 by the United States.
Donaldsonville is named after landowner William Donaldson. In 1806 Donaldson commissioned architect and planner Barthelemy Lafon to plan a new town. This served briefly as the Louisiana capital (1830–1831) after New Orleans was deemed "too noisy".