Los Alamos County (Spanish: The Cottonwoods) is a county in the U.S. state of New Mexico. As of the 2010 census, the population was 17,950. The Class H county-municipality was administered exclusively by the U.S. federal government during the Manhattan Project, but now has equal status to New Mexico's other counties. The county seat is Los Alamos. The other population center in the county is White Rock.
The county is home to the Los Alamos National Laboratory.
Los Alamos County is noted for having the lowest level of child poverty of any county in the United States.