Adult painted turtles can grow to 13–25 cm (5–10 in) long, with males being smaller. The shell is oval, smooth with little grooves where the large scale-like plates overlap, 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.

The hatchling has a proportionally larger head, eyes, and tail, and a more circular shell than the adult. The adult female is generally longer than the male, 10–25 cm (4–10 in) versus 7–15 cm (3–6 in). For a given length, the female has a higher (more rounded, less flat) top shell. The female weighs around 500 g (18 oz) on average, against the males' average adult weight of roughly 300 g (11 oz). The female's greater body volume supports her egg-production. The male has longer foreclaws and a longer, thicker tail, with the anus (cloaca) located further out on the tail.
The painted turtle has a very similar appearance to the red-eared slider (the most common pet turtle) and the two are often confused. The painted turtle can be distinguished because it is flatter than the slider. Also, the slider has a prominent red marking on the side of its head (the "ear") and a spotted bottom shell, both features missing in the painted turtle.

The most widespread North American turtle, the painted turtle is the only turtle whose native range extends from the Atlantic to the Pacific. It is native to eight of Canada's ten provinces, forty-five of the fifty United States, and one of Mexico's thirty-one states. On the East Coast, it lives from the Canadian Maritimes to the U.S. state of Georgia. On the West Coast, it lives in British Columbia, Washington, and Oregon and offshore on southeast Vancouver Island. The northernmost American turtle, its range includes much of southern Canada. To the south, its range reaches the U.S. Gulf Coast in Louisiana and Alabama. In the southwestern United States there are only dispersed populations. It is found in one river in extreme northern Mexico. It is absent in a part of southwestern Virginia and the adjacent states as well as in north-central Alabama. There is a harsher divide between midland and eastern painted turtles in the southeast because they are separated by the Appalachian mountains, but the two subspecies tend to mix in the northeast.

The borders between the four subspecies are not sharp, because the subspecies interbreed. Many studies have been performed in the border regions to assess the intermediate turtles, usually by comparing the anatomical features of hybrids that result from intergradation of the classical subspecies. Despite the imprecision, the subspecies are assigned nominal ranges.
The eastern painted turtle ranges from southeastern Canada to Georgia with a western boundary at approximately the Appalachians. At its northern extremes, the turtle tends to be restricted to the warmer areas closer to the Atlantic Ocean. It is uncommon in far north New Hampshire and in Maine is common only in a strip about 50 miles from the coast. In Canada, it lives in New Brunswick and Nova Scotia but not in Quebec or Prince Edward Island. To the south it is not found in the coastal lowlands of southern North Carolina, South Carolina, or Georgia, or in southern Georgia in general or at all in Florida.

In the northeast, there is extensive mixing with the midland subspecies, and some writers have called these turtles a "hybrid swarm". In the southeast, the border between the eastern and midland is more sharp as mountain chains separate the subspecies to different drainage basins.
The midland painted turtle lives from southern Ontario and Quebec, through the eastern U.S. Midwest states, to Kentucky, Tennessee and northwestern Alabama, where it intergrades with the southern painted turtle. It also is found eastward through West Virginia, western Maryland and Pennsylvania. The midland painted turtle appears to be moving east, especially in Pennsylvania. To the northeast it is found in wes…

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The painted turtle is a bottom-dwelling hunter. It quickly juts its head into and out of vegetation to stir potential victims out into the open water, where they are pursued. Large prey is ripped apart with the forefeet as the turtle holds it in its mouth. It also consumes plants and skims the surface of the water with its mouth open to catch small particles of food.

Although all subspecies of painted turtle eat both plants and animals (in the form of leaves, algae, fish, crustaceans, aquatic insects and carrion), their specific diets vary. Young painted turtles are mostly carnivorous and as they mature they become more herbivorous.

Painted turtles obtain coloration from carotenoids in their natural diet by eating algae and a variety of aquatic plants from their environment. Stripes and spots increase red and yellow chroma and decrease UV chroma and brightness in turtles with large amounts of carotenoids in their diet compared to the stripes and spots of turtles with only moderate amounts of carotenoids in their diet.
• The eastern painted turtle's diet is the least studied. It prefers to eat in the water, but has been observed eating on land. The fish it consumes are typically dead or injured.
• The midland painted turtle eats mostly aquatic insects and both vascular and non-vascular plants.
• The western painted turtle's consumption of plants and animals changes seasonally. In early summer, 60% of its diet comprises insects. In late summer, 55% includes plants. Of note, the western painted turtle aids in the dispersal of white water-lily seeds. The turtle consumes the hard-coated seeds, which remain viable after passing through the turtle, and disperses them through its feces.
Painted turtles are most vulnerable to predators when young. Nests are frequently ransacked and the eggs eaten by garter snakes, crows, chipmunks, thirteen-lined ground and gray squirrels, skunks, groundhogs, raccoons, badgers, gray and red fox, and humans. The small and sometimes bite-size, numerous hatchlings fall prey to water bugs, bass, catfish, bullfrogs, snapping turtles, three types of snakes (copperheads, racers and water snakes), herons, rice rats, weasels, muskrats, minks, and raccoons. As adults, the turtles' armored shells protect them from many potential predators, but they still occasionally fall prey to alligators, ospreys, crows, red-shouldered hawks, bald eagles, and especially raccoons.

Painted turtles defend themselves by kicking, scratching, biting, or urinating. In contrast to land tortoises, painted turtles can right themselves if they are flipped upside down.