Bladderwrack is a type of brown algae (seaweed) that grows on the northern Atlantic and Pacific coasts of the United States and on the northern Atlantic coast and Baltic coast of Europe. The main stem of bladderwrack, known as the thallus, is used medicinally. The thallus has tough, air-filled pods or bladders to help the algae float, thus the name bladderwrack. Although bladderwrack is sometimes called kelp, that name is not specific to this species and should be avoided.