Atwater is a city on U.S. Route 99 in Merced County, California, United States. Atwater is 8 miles (13 km) west-northwest of Merced, at an elevation of 151 feet (46 m). The population as of the 2010 census was 28,168.
According to the United States Census Bureau, the city has a total area of 6.1 square miles (15.8 km²). 99.86% of it is land and 0.14% water.
The city includes Castle Air Museum, but does not include Castle Air Force Base proper.