The Red-eared Terrapin (Trachemys scripta elegans), called the Red-eared Slider in the US, is native to the southern United States; and has become common in the UK.

It is a medium-sized aquatic animal similar in appearance and coloration to a tortoise, ranging in size from 2 cm (0.8 in) when born to 40 cm (16 in) as an adult, and is recognised by a red stripe down each side of the head. They are almost totally aquatic, only leaving the water to bask on hot sunny days; and they hibernate over the winter at the bottom of ponds or shallow lakes where they enter a state of torpor. Feeding on vegetation, insects and small fish, they are reasonably widespread in the UK, being more active during hot weather. They will tolerate other species in their habitat, but will quickly dive underwater when approached, making them difficult to catch.

Red-eared terrapins are not native to the British Isles; but have become common in most areas due to widespread release of imported pets by the public. People should realise that terrapins in captivity need a lot of care, as they can live to over 40 years old. They have a strong bite, resulting in injuries that should be seen by medical staff as soon as possible; and they may carry some diseases. They are not thought to be breeding in the UK, as the mean summer temperature is too low; however, there is some possibility that if temperatures rise by a few degrees breeding could start. Note: Recent news (2001) from a reptile specialist in the South (near Southampton) is that terrapin nests and eggs were found last year, but no hatchlings were found.

It is easy to determine the sex of an individual older than about a year. Males have longer front claws and a longer, thicker tail. Males also tend to have a concave ventral shell as compared to that of females. When full grown, females are also significantly larger than males.

There is a European turtle species, Emys orbicularis, the European Pond Turtle , similar in shape, size, and habit to the Red-eared Terrapin; but with a more rounded head. This species is not found in the UK, but is common in central and southern Europe. Red-eared terrapins are members of the order Testudines, which contains nearly 250 species. In the United States, members of this genus are usually referred to as turtles; however, in the UK they are split into Turtles (aquatic), Tortoises (land), and Terrapins (semi-aquatic).