Despite its name, the big brown bat is medium-sized, at about 13 cm (5 in.) long with a wingspan of 30 to 33 cm (12 to 13 in.). The soft fur is rather oily in texture and is brown. The wing and tail membranes are almost black and have a leathery texture. Other hairless skin on the ears and snout is also blackish.

This species has become well adapted to human habitation. They are commonly observed at dusk as they fly along lamp-lit streets in pursuit of the insects that are attracted to the lights. During the summer months, they often seek shelter behind loose boards, under eaves or shingles, in attics and church belfries, and so on. Many spend the winter months in caves or mine shafts, hanging in small clusters of up to 200 individuals. Because big brown bats also hibernate in the walls of our homes or in our sewers, they are usually the first species of bats seen in the spring.

The oldest known big brown bat was a male recorded at 19 years old. This is an astonishing age for such a small mammal. A mouse of comparable size would be deemed old at two years of age. Bats have few natural enemies.