Assumption Parish is a parish located in the U.S. state of Louisiana. As of the 2020 census, the population was 21,039. Its parish seat is Napoleonville. Assumption Parish was established in 1807, as one of the original parishes of the Territory of Orleans. Assumption Parish is one of the twenty-two Acadiana parishes. Its major product is sugar cane. In proportion to its area, Assumption Parish produces the most sugar of any parish of Louisiana.