The brook trout, Salvelinus fontinalis, (sometimes called the eastern brook trout) is a species of fish in the salmon family of order Salmoniformes. In many parts of its range, it is known as the speckled trout or squaretail. A potamodromous population in Lake Superior are known as coaster trout or, simply, as coasters. Though commonly called a trout, the brook trout is actually a char, along with lake trout, bull trout, Dolly Varden and the Arctic char.

The brook trout is native to small streams, creeks, lakes, and spring ponds. Some brook trout are anadromous. It is native to a wide area of eastern North America but increasingly confined to higher elevations southward in the Appalachian Mountains to northern Georgia, Canada from the Hudson Bay basin east, the Great Lakes–Saint Lawrence system, and the upper Mississippi River drainage as far west as eastern Iowa.[1] Typical pH range: 5 to 7.5 Unusual pH recordings: 3.5, 9.8[2]. Typical length: 25 to 65 cm (10 to 26 in). Typical weight: 0.3 to 3 kg (11 oz to 7 lb). Colouration: green to brown basic colouration with a distinctive marbled pattern (called vermiculations) of lighter shades across the flanks and back and extending at least to the dorsal fin, and often to the tail. There is a distinctive sprinkling of red dots, surrounded by blue haloes, along the flank. The belly and lower fins are reddish in color, the latter with white leading edges. Often the belly, particularly of the males, becomes very red or orange 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 (14 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.