Lake County is a county located in the U.S. state of Colorado. As of the 2020 census, the population was 7,436. The county seat and the only municipality in the county is Leadville. The highest natural point in Colorado and the entire Rocky Mountains is the summit of Mount Elbert in Lake County at 14,440 feet (4,400 meters) elevation.

Lake County was one of the original 17 counties created by the Colorado legislature on November 1, 1861. As originally defined, Lake County included a large portion of western Colorado to the south and west of its present boundaries. The county was named for Twin Lakes.

Placer gold was found at Colorado Gulch in 1863 as part of the Colorado Gold Rush.

Lake County slowly lost territory over the succeeding decades, losing land its southeast to Saguache County in 1866 and Hinsdale County in 1874; in its southwest to La Plata County in 1874 and San Juan County in 1876, and in its west to Ouray and Gunnison counties in 1877.

With its many reductions in size, Lake County's designated county seat also changed multiple times within just a few years, residing successively in Oro City (from 1861), Lourette (from 1863), Dayton (from 1866), and Granite (from 1868).

By 1878, Lake County had been reduced to an area including only present-day Lake and Chaffee counties. On February 8, 1879, the Colorado legislature renamed Lake County, Carbonate County. However, this designation name only lasted for two days, until Chaffee County was split off from Carbonate's southern section on February 10 and the remaining northern portion was redesignated Lake County with its current county seat of Leadville.

According to the U.S. Census Bureau, the county has a total area of 384 square miles (990 km²), of which 377 square miles (980 km²) is land and 7.0 square miles (18 km²) (1.8%) is water. It is the fourth-smallest county in Colorado by area.

As the courthouse with the highest elevation in the United States, the Lake County courthouse in Leadville is the "highest court" in the land.