A Becker naevus (nevus in American spelling) is a late-onset epidermal naevus or birthmark occurring mostly in males. It is also known as Becker melanosis. It is due to an overgrowth of the epidermis (upper layers of the skin), pigment cells (melanocytes) and hair follicles.

Rarely, like other birthmarks, there may be some abnormality of underlying tissues derived from the same embryonic cell type, the ectoderm. This is known as the Becker naevus syndrome, a type of epidermal naevus syndrome. These abnormalities may include:

• Smooth muscle hamartoma (overgrowth of smooth muscle tissue like a deep birthmark)
• Under-development of underlying structures such as breast, pectoral muscle, fat, limb, chest wall, spine
• Over-development of a tissue such as adrenal gland, limb, fingers or toes, scrotum