Donaldsonville is a small city in, and the parish seat of, Ascension Parish in south Louisiana, United States, located along the River Road of the west bank of the Mississippi River. The population was 7,436 at the 2010 census, a decrease of more than 150 from the 7,605 tabulation in 2000. Donaldsonville is part of the Baton Rouge Metropolitan Statistical Area.
Donaldsonville's historic district has what has been described as the finest collection of buildings from the antebellum era to 1933, of any of the Louisiana river towns above New Orleans. Union forces attacked the city, occupying it and several of the river parishes beginning in 1862. Fort Butler was built on the west bank of the Mississippi River. The fort was successfully defended on June 28, 1863, against a Confederate attack. This battle was one of the first occasions when free blacks and fugitive slaves fought as soldiers on behalf of the Union. The fort is listed on the National Register of Historic Places.
After the war, in 1868 Donaldsonville residents elected as mayor Pierre Caliste Landry, an attorney and Methodist minister; he was the first African American to be elected as mayor in the United States.