A terrapin is a type of small turtle that lives in fresh or brackish water. The term "terrapin" is not a taxonomic unit and can refer to different species of turtles depending on where they live. In general, terrapins are semi-aquatic, freshwater-living versions of turtles that live in either fresh or slightly salty (brackish) water. They are hard-shelled, egg-laying reptiles in the taxonomic order Chelonia, which includes turtles, tortoises, and terrapins.

Terrapins are omnivores and eat a variety of invertebrate prey and occasionally plant matter. They have interlocking shields or armor on their shells, which contain nerves and allow them to feel pressure and pain if their shell is broken or cracked.

Some examples of terrapins include the Indian pond terrapin, the Mexican spotted terrapin, and the Northern river terrapin. The diamondback terrapin is a type of terrapin that is the official state reptile of Maryland and the mascot of the University of Maryland, College Park.