Becker's nevus is a non-cancerous, large, brown birthmark occurring mostly in males. It can be present at birth, but is usually first noticed around puberty. It typically occurs on one shoulder and upper trunk but occasionally occurs elsewhere on the body. A Becker's nevus often becomes darker, and excessive hair growth (hypertrichosis) and acne may develop within the nevus.

Becker's nevus is due to overgrowth of the epidermis (upper layer of the skin), pigment cells (melanocytes) and hair follicles. The specific underlying cause is unknown. Because it often forms around puberty in males and is sometimes associated with acne and hair growth, its development may be triggered by androgens (male sex hormones such as testosterone).

Treatment is primarily for cosmetic reasons (hyperpigmentation or hair growth) and may include Ruby laser treatment or laser-assisted hair removal.

In very rare cases, Becker's nevus is associated with other skin features; muscular or skeletal features; or underdevelopment (hypoplasia) of the breast. When this occurs, the condition is known as Becker's nevus syndrome.

A Becker's nevus typically begins to develop around puberty on the shoulder or upper trunk, although it may develop on other areas of the body. It is sometimes present from birth. Pigmentation (darkening of skin color) may be subtle at first, but the nevus often darkens after puberty and expands. The resulting birthmark is usually large, brown, and on one side of the body. The nevus may grow more hair than the surrounding skin. In some cases, acne develops within the nevus.