Fort Saint Vrain, 1837 trading post near Platteville, Colorado, United States.

Fort Saint Vrain was an 1837 fur trading post of the Bent, St. Vrain Company at the confluence of Saint Vrain Creek and the South Platte River, about 20 miles east of the Rocky Mountains in present-day Weld County, Colorado. Its two-story adobe walls enclosed an interior courtyard, resembling the reconstructed adobe building and plaza at Fort Vasquez and Bent's Old Fort. Today only a historical marker at the end of Weld County Road 40 notes the site.

The Bent, St. Vrain Company received a trading license in 1836 and built the fort the following year to support the fur trade along the South Platte River. It operated for about a decade before shifting trade routes and the decline of the fur trade brought its commercial life to an end around 1848.

The fort is named after Ceran St. Vrain, a partner in the Bent, St. Vrain Company who helped establish it. His brother Marcellin managed the post and hired men such as James Beckwourth and Jean Baptiste Charbonneau, Sacagawea's son. Trade with Native American tribes and mountain men took place inside the adobe walls.

The site is marked at the end of Weld County Road 40, about seven miles north of Fort Vasquez and roughly 20 miles east of the Rocky Mountains. There is no building left, only the marker, so bring water and sun protection. A car is needed; the nearest paved route is Highway 85.

Jean Baptiste Charbonneau, the son of Sacagawea who traveled with the Lewis and Clark Expedition as a baby, later worked at this fort as a translator and trader. His presence here created a direct link between two very different chapters of early American western history.