Eureka is an extinct mining town in San Juan County, Colorado, United States, along the Animas River, between Silverton and Animas Forks. The town derives its name from the Greek interjection Eureka! The Eureka post office operated from August 9, 1875, until April 30, 1942.

San Juan County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 705, making it the least populous county in Colorado. The county seat and the only incorporated municipality in the county is Silverton. The county name is the Spanish language name for "Saint John", the name Spanish explorers gave to a river and the mountain range in the area. With a mean elevation of 11,240 feet, San Juan County is the highest county in the United States and also has the two highest elevation houses in the United States; the ‘Bonnie Belle’ above Animas Forks at 11,900’ – 11,950’ elevation and an unnamed house above Picayune Gulch at 12,000’ elevation.