The plastron of the male northern box turtle is slightly concave (curved inward), while the plastron of the female northern box turtle is flat. This is because the concave plastron allows the male to attach to the female's carapace without falling off during mating.

90% of males have red or orange irises , while female northern box turtles have brown or yellow irises. Female northern golden box turtles usually have a taller and more rounded shell, while male northern golden box turtles have a lower shell with a more oval shape.

In addition, the female's cloaca is rounder than the male's, more like a star, located closer to its body, and at the base of its tail, the male's cloaca is longer and more like a slit.