Ascidiacea (commonly known as the ascidians or sea squirts) is a class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide cellulose.

Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea and Larvacea swim freely like plankton, sea squirts are sessile animals: they remain firmly attached to substratum, such as rocks and shells.