Human skin is the largest organ by surface area.
An adult’s skin covers roughly 1.5–2 square metres and forms the body’s outer barrier. It helps limit water loss, blocks many pathogens, detects touch and temperature, and contributes to body-temperature regulation through sweating and changes in blood flow.
Skin has three main layers: the epidermis, dermis, and subcutaneous tissue. The epidermis is the visible outer layer; the dermis contains blood vessels, nerves, hair follicles, and many glands. The subcutaneous layer contains fat and connective tissue, although anatomists may not count it as part of the skin proper.
The liver is often called the largest internal organ, while the small intestine has a very large internal absorptive surface because of folds, villi, and microvilli. Those are different measurements, so they do not replace skin’s status as the largest organ by external surface area.