Workers are very tiny, 1/16 inch long, and monomorphic. The color is light tan to reddish with a darker abdomen. The antennae have 12 segments and a 3-segmented club, and the pedicel has 2 modes. The thorax is unevenly rounded.

Pharaoh ants have moderate to very large colonies (more than 100,000 workers) and a large number of queens per colony. New colonies are readily formed by budding, which makes their control particularly difficult (and proper identification critical). Although this is a warm-region ant that cannot tolerate temperate winters, colonies can thrive indoors in the Northeast. They will trail as far as 140 feet from the nest for food and water.

Pharaoh ant nest in inaccessible areas of a structure (including the upper floors of tall buildings) and trail to sources of food and water. They are particularly dangerous in hospitals and other healthcare settings because they tend to enter wounds and mouths of people in these facilities. They will trail on medical supplies and have been shown to carry and transmit pathogens. Traditional repellent insecticide sprays can split pharaoh ant colonies, worsening the problem.