Seaweed dermatitis is an irritant contact dermatitis resulting from a direct contact with Lyngbya majuscule, a common cyanobacterium found in many tropical, subtropical, and temperate waters around the world. Seaweed dermatitis is uncommon; outbreaks have mostly occurred during summer in the Hawaiian Islands, the Marshall Islands, Okinawa, Australia, and Florida.

Lyngbya majuscule is a blue-green alga that resides in the intertidal zone and forms threadlike filaments along the ocean floor. Lyngbya majuscule produces the dermatoxins lyngbyatoxin A and debromoaplysiatoxin, which are known to cause seaweed dermatitis upon exposure to the skin.

Seaweed dermatitis occurs after swimming in salt water and appears minutes to hours after exposure in a bathing suit distribution, most often in the genital and perianal area and the inframammary areas due to a bathing suit trapping the seaweed against the swimmer's body. It begins with stinging, itching, or a burning sensation that evolves into redness within minutes after exposure, followed by pustules and desquamation over the next few days. Severity of the dermatitis differs depending on the length of time of contact between the seaweed and skin. In severe cases, there may be bulla formation, weeping, and crusting.

There have been anecdotal reports of airborne contact dermatitis from aerosolized L majuscule toxins leading to respiratory irritation, eye inflammation, oral inflammation, and facial dermatitis.

Other associated symptoms include headache and fatigue. The dermatitis resolves completely in about a week, and treatment is tailored to provide symptomatic relief.