- Eureka is a city located in Humboldt County, California, United States. It is the county seat and largest city in the county, with a population of approximately 27,000 people. Eureka is known for its Victorian-style architecture, historic Old Town district, and its location on the shores of Humboldt Bay. The city is also home to several museums, art galleries, and outdoor attractions, including the nearby Redwood National and State Parks. Eureka is often referred to as the "Gateway to the Redwoods" due to its proximity to the ancient and towering redwood trees that thrive in the region.