Mad River is a census-designated place in Trinity County, California. Mad River is located in the southern part of the county. Mad River sits at an elevation of 2,484 feet. The ZIP Code is 95552. Its population is 361 as of the 2020 census, down from 420 from the 2010 census. Mad River has the last services on California State Route 36 until the Platina Store, 44 miles away. Until it closed, the next services were the Wildwood Store, 5 miles west of Platina.