The Drosophila melanogaster accessory gland is a functional analog of the mammalian prostate made up of two secretory epithelial cell types, termed main and secondary cells.This tissue is responsible for making and secreting seminal fluid proteins and other molecules that contribute to successful reproduction. Here, we show that similar to the mammalian prostate, this tissue grows with age. We find that the adult accessory gland grows in part via endocycles to increase DNA content and cell size. The differentiated, binucleated main cells remain poised to endocycle in the adult gland, and tissue damage or upregulation of signals that promote growth are sufficient to trigger dramatic endocycling leading to increases in cell size and ploidy, independent of mating status. Our data establishes that the adult accessory gland is not quiescent, but instead uses endocycles to maintain accessory gland function throughout the fruit fly's lifespan.