The painted turtle's shell is 10–25 cm (4–10 in) long, oval, smooth, and flat-bottomed.  The color of the top shell (carapace) varies from olive to black.

Darker specimens are more common where the bottom of the water body is darker. The bottom shell (plastron) is yellow, sometimes red, sometimes with dark markings in the center. Similar to the top shell, the turtle's skin is olive to black, but with red and yellow stripes on its neck, legs, and tail.  As with other pond turtles, such as the bog turtle, the painted turtle's feet are webbed to aid swimming.

The head of the turtle is distinctive. The face has only yellow stripes, with a large yellow spot and streak behind each eye, and on the chin two wide yellow stripes that meet at the tip of the jaw.  The turtle's upper jaw is shaped into an inverted "V" (philtrum), with a downward-facing, tooth-like projection on each side.