El Molino Viejo, also known as The Old Mill, is a former grist mill in the San Rafael Hills of present-day San Marino, California, United States, and was built in 1816 by Father José María de Zalvidea from the Mission San Gabriel Arcángel (San Gabriel Mission). It is the oldest commercial building in Southern California, and was one of the first ten sites in Los Angeles County to be listed on the National Register of Historic Places, receiving the recognition in 1971. The Old Mill has also been designated as a California Historical Landmark.