The general color of the eastern gartersnake is variable; it may be blackish, dark brown, greenish, or olive. There are usually three yellowish stripes, one down the back and one along each side. The area between the stripes on each side may have a double row of alternating dark spots. The belly is yellowish green with two rows of faint black spots that are indistinct, somewhat hidden by the overlapping scales. Scales on the back are keeled; the anal plate is single. When cornered, this snake often flattens its head and body and tries to strike. Like many kinds of harmless snakes, gartersnakes will bite to defend themselves.
Another subspecies, the red-sided gartersnake (Thamnophis sirtalis parietalis), is found in the western half of the state. It has some degree of red or orange-red skin that shows along its sides.