Fort Saint Vrain was an 1837 fur trading post built by the Bent, St. Vrain Company, and located at the confluence of Saint Vrain Creek and the South Platte River, about 20 miles (32 km) east of the Rocky Mountains in the unorganized territory of the United States. This area later became part of the State of Colorado. A historical marker notes the place where Old Fort St. Vrain once stood, today at the end of Weld County Road 40, located about seven miles north of Fort Vasquez, Colorado. Among those who helped to establish the fort was Ceran St. Vrain, after whom it was named.