The St. Marys River is a 126-mile-long (203 km) river in the southeastern United States. The river was known to the Indians of the area as Thlathlothlaguphka, or Phlaphlagaphgaw, meaning "rotten fish". French explorer Jean Ribault named the river the Seine when he encountered it in 1562. From near its source in the Okefenokee Swamp, to its mouth at the Atlantic Ocean, it forms a portion of the border between the U.S. states of Georgia and Florida. The river also serves as the southernmost point in the state of Georgia. The St. Marys River rises as a tiny stream, River Styx, flowing from the western edge of Trail Ridge, the geological relic of a barrier island/dune system, and into the southeastern Okefenokee Swamp. Arching to the northwest, it loses its channel within the swamp, then turns back to the southwest and reforms a stream, at which point it becomes the St. Marys River. Joined by another stream, Moccasin Creek, the river emerges from the Okefenokee Swamp at Baxter, Florida/Moniac, Georgia. It then flows south, then east, then north, then east-southeast intersecting I-95 near Yulee, and finally emptying its waters into the Atlantic, near St. Marys, Georgia and Fernandina Beach, Florida.

Yulee is a Census County Division (CCD) located within Nassau County, Florida, in the United States. Positioned in northeastern Florida, Yulee lies directly south of the Florida-Georgia state line, marked by the St. Marys River. Furthermore, it is situated 24 miles north of downtown Jacksonville, Florida. As of the latest survey conducted by the U.S. Census Bureau in 2021, the CCD's population is recorded at 36,742. Yulee is a constituent of the Jacksonville metropolitan area, which encompasses an estimated population of approximately 1.68 million individuals as of 2022. Presently, the Yulee Census County Division serves as a residential bedroom community for individuals commuting to Jacksonville, Naval Submarine Base Kings Bay, and various locations in Southeast Georgia. It is also actively engaged in efforts to broaden its tax base for economic diversification.