Tsuga mertensiana, known as mountain hemlock, is a species of hemlock native to the west coast of North America, found between Southcentral Alaska and south-central California.

Tsuga mertensiana is a large evergreen conifer growing up to 20 to 40 meters (66 to 131 feet) tall, with exceptional specimens as tall as 59 m (194 ft) tall. They have a trunk diameter of up to 2 m (6+1⁄2 ft). The bark is about 3 centimeters (1+1⁄4 inches) thick and square-cracked or furrowed, and purplish-brown to gray in color. The crown is a neat, slender, conic shape in young trees with a tilted or drooping lead shoot, becoming cylindric in older trees. At all ages, it is distinguished by the slightly pendulous branchlet tips. The shoots are orange–brown, with dense pubescence about 1 millimeter (1⁄16 in) long. The leaves are needle-like, 7 to 25 mm (1⁄4 to 1 in) long and 1 to 1.5 mm (1⁄32 to 1⁄16 in) broad, soft, blunt-tipped, only slightly flattened in cross-section, pale glaucous blue-green above, and with two broad bands of bluish-white stomata below with only a narrow green midrib between the bands; they differ from those of any other species of hemlock in also having stomata on the upper surface, and are arranged spirally all around the shoot.

The cones are small (but much longer than those of any other species of hemlock), pendulous, cylindrical, 3 to 8 cm (1 to 3 in) long and 8 to 10 mm (5⁄16 to 3⁄8 in) broad when closed, opening to 12 to 35 mm (1⁄2 to 1+3⁄8 in) broad, superficially somewhat like a small spruce cone. They have thin, flexible scales 8 to 18 mm (5⁄16 to 11⁄16 in) long. The immature cones are dark purple (rarely green), maturing red–brown 5 to 7 months after pollination. The seeds are red–brown, 2 to 3 mm (1⁄16 to 1⁄8 in) long, with a slender, 7 to 12 mm (1⁄4 to 1⁄2 in)-long pale pink–brown wing.

There are three taxa, two subspecies and a minor variety:

The species name mertensiana refers to Karl Heinrich Mertens (1796–1830), a German botanist who collected the first specimens as a member of a Russian expedition in 1826–1829.

The distribution of T. mertensiana stretches from Kenai Peninsula, Alaska, to northern Tulare County, California. Its range fairly closely matches that of T. heterophylla (western hemlock), found less than 100 kilometers (62 miles) from the Pacific Ocean, apart from an inland population in the Rocky Mountains in southeast British Columbia, northern Idaho, and western Montana. The inland populations most likely established after deglaciation by remarkable long-distance dispersal of more than 200 km (120 mi) from the coast populations. Their ranges differ in California, where western hemlock is restricted to the Coast Ranges and mountain hemlock is found in the Klamath Mountains and Sierra Nevada. Unlike western hemlock, mountain hemlock mostly grows at high altitudes except in the far north, from sea level to 1,000 m (3,300 ft) in Alaska, 1,600 to 2,300 m (5,200 to 7,500 ft) in the Cascades in Oregon, and 2,500 to 3,050 m (8,200 to 10,010 ft) in the Sierra Nevada.

Mountain hemlock is usually found on cold, snowy subalpine sites where it grows slowly, sometimes attaining more than 800 years in age. Arborescent individuals that have narrowly conical crowns until old age (300 to 400 years) and shrubby krummholz on cold, windy sites near timberline add beauty to mountain landscapes. Areas occupied by mountain hemlock generally have a cool to cold maritime climate that includes mild to cold winters, a short, warm-to-cool growing season and moderate to high precipitation.

Best development of mountain hemlock is on loose, coarse-textured, well-drained soils with adequate moisture, and in British Columbia, on thick and very acidic organic matter and decayed wood. Adequate soil moisture appears to be especially important in California and Montana, where summer drought is most pronounced.