Males and females appear identical with a black crown, white forehead, gray back, gray wings above with white below, orange legs and a black-tipped yellow bill.
Immature birds have darker feathers, a dark bill and dark eye stripes on white heads.
Nesting success depends on the presence of bare or nearly barren sandbars, favorable water levels during nesting and abundant food. The nest is an inconspicuous, unlined scrape usually containing three brown spotted eggs. Egg-laying and incubation occur from late May through early August. Eggs hatch in about 20 days and chicks are fledged in about another 20 days. The interior least tern feeds on small fish and crustaceans taken by diving from the air into shallow water.