The Fort Union National Monument is a US National Monument and historic area in Watrous, New Mexico. It is named after Fort Union, the second fortification site built to serve as a guardian of the Santa Fe Trail.

Initially built in 1851, Fort Union was established three times in the area. The third one was the largest in the American Southwest region. It served as a military supply depot, territorial arsenal, and a military garrison.
Today, the Fort Union National Monument serves as a living testament to the events of the past that helped shape the country. Its adobe ruins serve as proof to the times past, when the United States was still being settled into by migrants.

At the site, guests can find a visitor center featuring exhibits about the Santa Fe Trail. The largest network of ruts of the old trail is also visible from the site.