The Klamath Mountains are a rugged and lightly populated mountain range in northwestern California and southwestern Oregon in the western United States. As a mountain system within both the greater Pacific Coast Ranges and the California Coast Ranges, the Klamath Mountains have a varied geology, with substantial areas of serpentinite and marble, and a climate characterized by moderately cold winters with very heavy snowfall and warm, very dry summers with limited rainfall, especially in the south. As a consequence of the geology and soil types, the mountains harbor several endemic or near-endemic trees, forming one of the largest collections of conifers in the world. The mountains are also home to a diverse array of fish and animal species, including black bears, large cats, owls, eagles, and several species of Pacific salmon. Millions of acres in the mountains are managed by the United States Forest Service. The northernmost and largest sub-range of the Klamath Mountains are the Siskiyou Mountains.

Physiographically, the Klamath Mountains include the Siskiyou Mountains, the Marble Mountains, the Scott Mountains, the Trinity Mountains, the Trinity Alps, the Salmon Mountains, and the northern Yolla-Bolly Mountains. They are a section of the larger Pacific Border province, which in turn is part of the Pacific Mountain System (Pacific Coast Ranges) physiographic division. Klamath Mountains is the name given to one of California's eleven geomorphic provinces.

These are the ten highest points in the Klamath Mountains:

A large portion of the Klamath Mountains is managed by the United States Forest Service. Several national forests lie in the Klamath Mountains region, including the Shasta-Trinity National Forest, Siskiyou National Forest, Klamath National Forest, Six Rivers National Forest, and Mendocino National Forest.

The Klamath Mountains contain 11 wilderness areas in both Oregon and California:

There are extensive hiking trail systems, recreation areas, and campgrounds both primitive and developed in the Klamaths. A 211-mile (340 km) stretch of the Pacific Crest Trail (PCT) passes through these mountains as well. This section of the PCT is known locally as "The Big Bend" and is the transition from the California Floristic Province to the Cascades.

The Bigfoot Trail is a 400-mile (640 km) trail through the Klamath Mountains from the Yolla Bolly-Middle Eel Wilderness to Crescent City, California.

The rocks of the Klamath Mountains originated as island arcs and continental fragments in the Pacific Ocean. The island masses consisted of rifted fragments of pre-existing continents and volcanic island masses created over subduction zones. These island masses contain rocks as old as 500 million years, dating to the early Paleozoic Era. A succession of eight island terranes moved eastward on the ancient Farallon Plate and collided with the North American Plate between 260 and about 130 million years ago. Each accretion left a terrane of rock of a single age. During the accretion, subduction of the plate metamorphosed the overlying rock and produced magma which intruded the overlying rock as plutons. Serpentinite, produced by the metamorphism of basaltic oceanic rocks, and intrusive rocks of gabbroic to granodiorite composition are common rocks within the Klamath terranes.