Brown recluse spiders are usually between 6–20 mm (1⁄4 in and 3⁄4 in), but may grow larger. While typically light to medium brown, they range in color from cream-colored to dark brown or blackish gray. These spiders usually have markings on their cephalothorax, with a dark line coming from it that looks like a violin, resulting in the nicknames fiddleback spider.

Adult brown recluse spiders often live about one to two years. Each female produces several egg sacs over a period of two to three months, from May to July, with approximately fifty eggs in each sac. The eggs hatch in about one month. The spiderlings take about one year to grow to adulthood. The brown recluse spider is resilient, and can tolerate up to six months of extreme drought and scarcity or absence of food.

Brown recluse spiders build irregular webs that frequently include a shelter consisting of disorderly thread. They frequently build their webs in woodpiles and sheds, closets, garages, plenum spaces, cellars, and other places that are dry and generally undisturbed. They have also been encountered in shoes, inside dressers, in bed sheets of infrequently used beds, in clothes stacked or piled or left lying on the floor, inside work gloves, behind baseboards and pictures, in toilets, and near sources of warmth when ambient temperatures are lower than usual.