Los Alamos County is a county in the U.S. state of New Mexico. As of the 2020 census, the population was 19,419. The smallest county by area in the state, Los Alamos County was formerly administered exclusively by the U.S. federal government during the Manhattan Project, but it now has equal status to New Mexico's other counties. The county has two census-designated places: Los Alamos and White Rock.

Los Alamos is a census-designated place in Los Alamos County, New Mexico, United States, that is recognized as one of the development and creation places of the atomic bomb—the primary objective of the Manhattan Project by Los Alamos National Laboratory during World War II. The town is located on four mesas of the Pajarito Plateau, and had a population of about 19,200 as of 2022. It is the county seat and one of two population centers in the county known as census-designated places (CDPs); the other is White Rock.