Physalia is a genus of the order Siphonophorae, colonies of four specialized polyps and medusoids that drift on the surface of the Atlantic, Indian and Pacific oceans.

A sail on the float, which may be left or right-handed, propels Physalia about the sea, often in groups.

These siphonophores sometimes become stranded on beaches, where their toxic nematocysts can remain potent for weeks or months in moist conditions.

Both species of this siphonophore resemble a jellyfish in appearance, with their gas-filled float and cluster of polyps beneath, which can hang up to 30 or 165 ft (9 to 50 m) below the surface of the sea.

Because it frequently washes up on beaches on the coast of the Atlantic, Indian and Pacific Oceans and is seen in the open ocean, Physalia is the best-known genus of siphonophore, as nearly all siphonophores live in the cold, dark ocean depths where they can only be observed in their natural habitat by a submersible or ROV.