The brown trout is a medium-sized fish, growing to 20 kg (44 lb) or more and a length of about 100 cm (39 in) in some localities, although in many smaller rivers, a mature weight of 1.0 kg (2.2 lb) or less is common. S. t. lacustris reaches an average length of 40–80 cm (16–31 in) with a maximum length of 140 cm (55 in) and about 60 pounds (27 kg).

The spawning behaviour of brown trout is similar to that of the closely related Atlantic salmon. A typical female produces about 2,000 eggs per kg (900 eggs per lb) of body weight at spawning. On September 11, 2009, a 41.45-lb (18.80-kg) brown trout was caught by Tom Healy in the Manistee River system in Michigan, setting a new state record. As of late December 2009, the fish captured by Healy was confirmed by both the International Game Fish Association and the Fresh Water Fishing Hall of Fame as the new all-tackle world record for the species. This fish now supplants the former world record from the Little Red River in Arkansas.