Tunicates, previously known as Urochordata, or urochordates, are members of the Tunicata, a subphylum of the phylum Chordata. They are marine filter feeders with a saclike morphology. In their respiration and feeding they take in water through an incurrent (or inhalant) siphon and expel the filtered water through an excurrent (or exhalant) siphon. Most adult tunicates are sessile and attached to rocks or similarly suitable surfaces on the ocean floor; others such as salps, doliolids and pyrosomes swim in the pelagic zone as adults. Various species are commonly known as sea squirts or sea pork.

The Tunicata apparently evolved in the early Cambrian period. Despite their simple appearance, they are a sister subphylum to the Vertebrata.