Castaic is an unincorporated community in the northwestern part of Los Angeles County, California, United States. As of the 2010 census it had a population of 19,015. For statistical purposes the Census Bureau has defined Castaic as a census-designated place. Tens of thousands of motorists pass through Castaic daily as they drive to or from Los Angeles on Interstate 5. Castaic Lake is part of the California Water Project and is the site of a hydro-electric power plant.