Fort L'Huillier was a short-lived fortification located on the Blue Earth River, just south of its confluence with the Minnesota River in southern Minnesota, United States. It was named in honor of a French chemist L'Huillier and was built in the fall and winter of 1700-01 by Pierre-Charles Le Sueur, a French trader and explorer interested in mining a blue clay that he thought was copper ore. The original garrison held about 20 men. Le Sueur left the fort in 1701 to take two tons of clay to New Orleans for further analysis. While he was gone, however, the fort was apparently attacked and abandoned by 1702, and no more was heard of the remaining men. In addition, the blue clay was found to be worthless, not the copper ore Le Sueur had hoped.