Blanding's turtle (Emydoidea blandingii) is a species of semi-aquatic turtle of the family Emydidae. This species is native to central and eastern parts of Canada and the United States. It is considered to be an endangered species throughout much of its range. Blanding's turtle is of interest in longevity research, as it shows few or no common signs of aging and is physically active and capable of reproduction into eight or nine decades of life.

There are differences of opinion as to the genus for this species; both Emys and Emydoidea occur in published sources in 2009, 2010, and 2011.

Both the specific name, blandingii, and the common name, Blanding's turtle, are in honor of American naturalist Dr. William Blanding (1773–1857). The Blanding’s turtle is also known as “the turtle with the sun under its chin,” a name that comes from Indigenous oral traditions. In one Indigenous creation story, the turtle is said to have rescued the sun and returned it to the sky by carrying it in its mouth. This left the turtle with the permanent glow of the sun on its neck and chin.

Blanding's turtle is a medium-sized turtle with an average straight carapace length of approximately 18 to 23 cm (7.1 to 9.1 in) with a maximum of 25.5 cm (10.0 in). A distinguishing feature of this turtle is the bright yellow chin and throat. The carapace, or upper shell, is domed, but slightly flattened along the midline, and is oblong when viewed from above. The carapace is speckled with numerous yellow or light-colored flecks or streaks on a dark background. The plastron, or lower shell, is yellow with dark blotches symmetrically arranged. The head and legs are dark, and usually speckled or mottled with yellow. Blanding's turtle is also called the "semi-box" turtle, for although the plastron is hinged, the plastral lobes do not shut as tight as the box turtles'.

Blanding's turtle takes 14–20 years to reach sexual maturity. Mating begins between March and April and nests are dug from late May into early July . Clutch size varies from region to region. In New York, the clutch size ranges from five to twelve eggs, with an average of eight. The sex of the hatchlings is determined by the temperature of the eggs during development. Colder temperatures produce males, while warmer temperatures produce female hatchlings.

Blanding's turtle overwinters under or near water, in mud, or under vegetation or debris. This is known as brumation. During the nesting season, a female Blanding's turtle may be found more than a kilometer from where it hibernated. It is omnivorous, eating crustaceans (including crayfish), insects (such as dragonfly nymphs and aquatic beetles), snails and other invertebrates, fish, fish eggs, frogs, carrion, berries, seeds, and vegetable debris. It will eat coontail, duckweed, sedge, and bulrush. It is capable of catching live fish. Based on the extreme lack of aging symptoms and lack of age related decline, this turtle is considered a negligibly senescent species.

Blanding's turtle is a timid turtle and may plunge into water and remain on the bottom for hours when alarmed. If away from water, the turtle will withdraw into its shell. It is very gentle and rarely attempts to bite in defence. It is very agile and a good swimmer.

The geographic range of E. blandingii centres on the Great Lakes, and extends from central Nebraska and Minnesota (where it twice failed to become the state reptile) eastward through southern Ontario and the south shore of Lake Erie as far east as northern New York. In Nebraska, this turtle is uncommon in the eastern portion of the state, but common to abundant in the Sand Hills region lakes, ponds, and streams. There are also isolated populations in southeastern New York (Dutchess County), New England, and Nova Scotia.