Klamath County was a county of California from 1851 to 1874. During its existence, the county seat moved twice and ultimately portions of the territory it once had were carved up and added to nearby counties. It was formed from the northwestern portion of Trinity County, and originally included all of the northwestern part of the state, from the Mad River in the south to Oregon in the north, from the Pacific Ocean in the west to the middle of what is now Siskiyou County in the east. It is the only county in California to be disestablished.

Trinity County is a county located in the northwestern portion of the U.S. state of California. Trinity County is rugged, mountainous, heavily forested, and lies along the Trinity River within the Salmon and Klamath Mountains. It is also one of three counties in California with no incorporated cities.