Adult boxelder bugs have a black body and a red-orange color that outlines their wings, sides, and thorax. They are winged and capable of flight. When at rest, their wings lay flat across their backs.

Adults grow to about ½ of an inch in length. They have oval shaped bodies, specialized sucking mouthparts for feeding, and their head is much narrower than the rest of their body. Young boxelder bugs look like little tiny adults except that they are solid red and have yet to develop wings.