Pinus longaeva, the Great Basin Bristlecone Pine, is a long-living species of tree found in the higher mountains of the southwest United States. The species is one of three closely related trees known as bristlecone pines and is sometimes known as the Intermountain or Western bristlecone pine. One member of this species, known as "Methuselah", is the oldest known living non-clonal organism on Earth.

It is a medium-size tree, reaching 5 to 15 m (16 to 49 ft) tall and with a trunk diameter of up to 2.5 to 3.6 m (8 ft 2 in to 11 ft 10 in). The bark is bright orange-yellow, thin and scaly at the base of the trunk. The leaves ('needles') are in fascicles of five, stout, 2.5 to 4 cm (0.98 to 1.6 in) long, deep green to blue-green on the outer face, with stomata confined to a bright white band on the inner surfaces. The leaves show the longest persistence of any plant, with some remaining green for 45 years (Ewers & Schmid 1981).

These ancient trees have a gnarled and stunted appearance, especially those found at high altitudes, and have reddish-brown bark with deep fissures. As the tree ages, much of its vascular cambium layer may die. In very old specimens, often only a narrow strip of living tissue connects the roots to a handful of live branches.