Though classified as carnivores, black bears have an omnivorous diet. In spring they consume emerging plants and carcasses of animals that have died during the winter. Fruits dominate the diet in summer, and both fruit and mast, especially acorns and beechnuts, constitute most of the fall diet. As opportunistic feeders, black bears will also eat pinecones, roots, ants, and honey from wild or domestic bees. Nonetheless, black bears are strong predators, and in some areas they frequently kill moose calves and deer fawns during spring. Black bears living near humans adapt readily to alternate food sources, such as garbage from dumps or campsites and handouts from tourists in parks. Human encounters with black bears occasionally result in injury or death, and attacks are reported every year. In almost all cases, avoiding surprise encounters is the best defense, as black bears prefer to avoid people.

Throughout most of their range, black bears become dormant during winter. They spend the winter in dens located in rock crevices, in underground burrows, under tree roots, in hollow trees, in brush piles, or simply on open-ground beds. Prior to winter sleep, bears must accumulate large quantities of body fat during late summer and fall. Not only does this enable them to survive the long period of winter fasting, but it also allows them to have sufficient energy in spring when they emerge and food is rare. For females, the amount of fat stored before winter is linked with reproductive success: fatter females typically have more and bigger young than do leaner females. Accumulating fat for the winter is thus a strong drive, and it explains the constant search for food through the summer and fall.

Black bears are not territorial; they are mostly solitary, and the home ranges of both males and females may overlap. Home ranges typically are larger where food is less abundant and smaller where food is plentiful. Throughout Canada and the United States, home ranges of black bears extend from about 40 to 200 square km (15 to 77 square miles) for males but are considerably smaller for females. Breeding begins in the spring and peaks during June and July. Black bears are promiscuous, males and females often mating with several individuals. Implantation of the fertilized egg is delayed, occurring in November or December. Actual gestation then lasts 60–70 days, and one to four young cubs are born in January or February. Born blind, fully furred, and toothless, the cubs remain with the mother for 16 months, and the female breeds every second or third year. Although the mother is very protective of her litter, young cubs may be killed by coyotes, wolves, brown bears, or other black bears. Black bears can live for more than 20 years in the wild, but in areas near human habitation most black bears die sooner as a result of hunting, trapping, poaching, nuisance removal near campgrounds or dumps, and collision with vehicles.

In natural habitats, black bears are active during the day. However, in areas of high human activity such as dumps and campsites, black bears often become nocturnal to avoid encounters with humans. Even so, black bears habituate quickly to handouts given by tourists, and this lack of fear of humans often leads to conflicts. In parks, human-habituated bears often must be killed as they become hazardous around campsites. Not feeding wild black bears is therefore better for both humans and bears. The black bear may be tamed and taught various tricks, and it is a common performer in circuses and other animal acts.

The Asiatic black bear (Ursus thibetanus) is similar to the American black bear in size and appearance.