Eureka, California. For the former settlement in Nevada County, see Eureka, Nevada County, California. For other uses, see Eureka (disambiguation). Not to be confused with Yreka, California. Eureka (Hupa: do'-wi-lotl-ding' Karuk: uuth) is the principal city and county seat of Humboldt County in the Redwood Empire region of California. The city is located on U.S. Route 101 on the shores of Humboldt Bay, 270 miles (430 km) north of San Francisco and 100 miles (160 km) south of the Oregon border.