The listing of the distinct population segment of Acipenser oxyrinchus oxyrinchus (Atlantic Sturgeon) found within Chesapeake Bay as “endangered” under the Endangered Species Act in 2012 spurred new research efforts that recorded its presence in many major tributaries of Chesapeake Bay. However, the potential use of former habitat in the Patuxent River has received little attention. We describe the movement of a mature male Atlantic Sturgeon in the Patuxent River detected using passive acoustic telemetry. The sturgeon was originally tagged in the James River, VA, on 7 September 2013 and was detected in the Patuxent River, MD, between 18 September and 16 October 2016, which coincides with the fall spawning season in Virginia tributaries. This report represents the first documented occurrence of an Atlantic Sturgeon in the Patuxent River since 1997 and the first verified mature individual since 1924. It also details the movement of an adult sturgeon into freshwater reaches of a non-natal river, an unusual behavior for this species. We discuss the behavioral and environmental context of our observations, and highlight the value of acoustic telemetry in documenting the presence of endangered aquatic species for conservation and management.