The brook trout (Salvelinus fontinalis) is a species of freshwater fish in the salmon family Salmonidae. It is native to eastern North America in the United States and Canada, but has also been artificially introduced to other parts of North America and other continents. In parts of its range, it is also known as eastern brook trout, speckled trout, Brook charr, square-tailed trout or mud Trout, among others. A potamódroma population in Lake Superior is known as mountain trout or simply mountain trout.

Brook trout have a dark green to brown color, with a distinctive veined pattern (called vermiculation) of lighter tones on the flanks and back and extending at least to the dorsal fin and often to the tail. A distinctive pinch of red dots, surrounded by blue halos, occurs along the flanks. The belly and lower fins are reddish, the latter with white leading edges. Often the belly, particularly of males, becomes very red or orange when the fish are spawning.

Typical brook trout lengths range from 25 to 65 cm (9.8 to 25.6 inches) and weights from 0.3 to 3 kg (0.66 to 6.61 pounds). The maximum recorded length is 86 cm (34 inches) and the maximum weight is 6.6 kg (15 pounds). Brook trout can reach at least seven years of age, with reports of 15-year-old specimens observed in California habitats where the species has been introduced. Growth rates depend on Season, age, Air and water temperatures, and flow rates. In general, flow rates affect the rate of change in the relationship between temperature and growth rate. For example, in spring, growth increased with temperature at a faster rate with high flow rates than with low flow rates.