The brook trout (Salvelinus fontinalis) is a species of anadromous fish in the salmon family (family Salmonidae) of order Salmoniformes. One of the trouts, it is native to a wide area of eastern North America, including most of Canada from the Hudson Bay basin east, the Great Lakes–Saint Lawrence system, and the Mississippi River drainage in the United States as far south as northern Georgia.

The brook trout is of dark green to brown basic colouration with a distinctive marbled pattern of lighter shades across the flanks and back and extending to the dorsal fin and often the tail. The belly and lower fins are reddish in coulour, the latter with white leading edges. The lower fish becomes very red when the fish are spawning. The species reaches a maximum recorded length of 86 cm (33 in) and a maximum recorded weight of 9.4 kg (21 lb). It can reach at least seven years of age, with reports of 15-year-old specimens observed in California habitats to which the species has been introduced.

S. fontinalis prefers cool, clear waters in lakes, rivers, and streams, being sensitive to poor oxygenation. Its diverse diet includes crustaceans, frogs and other amphibians, insects, molluscs, smaller fish, and even small aquatic mammals such as voles. It provides food for seabirds and suffers attack by lampreys.

The species normally spends its entire life in fresh water, but some individuals—colloquially called "salters"—spend up to three months at sea in the spring, remaining within a few kilometres of river mouths. It returns upstream to spawn in the autumn or late summer, the female burying her eggs in a depression and leaving them; the eggs hatch in approximately 100 days.

The brook trout is very popular with anglers. It is also raised in large numbers for commercial food production, being sold for human consumption both fresh and smoked. It is also used for scientific experimentation.

Because of its popularity as a gamefish, the brook trout has been introduced and become established widely throughout the world. It has often had a harmful effect on native species, and is a potential pest. Nonetheless, the Brown Trout, a species not native to North America, has replaced the brook trout in much of the brook trout's native water.

The specific epithet fontinalis derives from the Latin fontīnālis (of or from a spring or fountain).