Humboldt County (/ˈhʌmboʊlt/ ^ⓘ) is the westernmost county in the U.S. state of California. As of the 2020 census, the population was 136,463. The county seat is Eureka.

Humboldt County comprises the Eureka–Arcata–Fortuna, California, Micropolitan Statistical Area. It is located on the far North Coast of California, about 270 miles (435 km) north of San Francisco. It has among the most diverse climates of United States counties, with very mild coastal summers and hot interior days. Similar to the greater region, summers are extremely dry and winters have substantial rainfall.

Its primary population centers of Eureka, the site of College of the Redwoods main campus, and the smaller college town of Arcata, site of California State Polytechnic University, Humboldt, are located adjacent to Humboldt Bay, California's second largest natural bay. Area cities and towns are known for hundreds of ornate examples of Victorian architecture.

Humboldt County is a densely forested mountainous and rural county with about 110 miles (177 km) of coastline (more than any other county in the state), situated along the Pacific coast in Northern California's rugged Coast (Mountain) Ranges. With nearly 1,500,000 acres (6,100 km²) of combined public and private forest in production, Humboldt County alone produces twenty percent of the total volume and thirty percent of the total value of all forest products produced in California. The county contains over forty percent of all remaining old growth Coast Redwood forests, the vast majority of which are protected or strictly conserved within dozens of national, state, and local forests and parks, totaling approximately 680,000 acres (1,060 sq mi).

The original inhabitants of the area now known as Humboldt County include the Algic Wiyot, Yurok; the Hokan Karuk; and the Athapaskan Hupa, Chilula, Whilkut, Tsnungwe as well as the Eel River Athapaskan peoples, including the Wailaki, Mattole and Nongatl.

Spanish traders made unintended visits to California with the Manila Galleons on their return trips from the Philippines beginning in 1565. The first recorded entry by people of European origin was a landing by the Spanish in 1775 in Trinidad.

The first recorded entry of Humboldt Bay by non-natives was an 1806 visit from a sea otter hunting party from Sitka employed by the Russian-American Company. The party included New England captain Jonathan Winship, Russian supervisors, and Aleut hunters. The bay was not visited again by people of European origin until 1849 when Josiah Gregg's party arrived. The following year Douglas Ottinger and Hans Buhne sailed the schooner Laura Virginia into the bay. One of the pioneering passengers, Steven W. Shaw, proposed that the Laura Virginia Association name the bay Humboldt in honor of the naturalist and explorer Alexander von Humboldt. The name was later applied to the county as a whole.

The area around Humboldt Bay was once solely inhabited by the Wiyot Indian tribe. One of the largest Wiyot villages, Tolowot, was located on Indian Island in Humboldt Bay. Founded around 900 BC, it contains a shell midden 6 acres (2.4 ha) in size and 14 feet (4.3 m) deep. It was the site of the February 26, 1860, massacre of the Wiyot people that was recorded by Bret Harte, then living in Union, now called Arcata. Between 60 and 200 Wiyot men, women, and children were murdered that night in the midst of a religious ceremony. Tolowot is now a restricted site and a National Historic Landmark. In 2019, the island was restored to the Wiyot tribe, and is now known as Tuluwat or Duluwat island.