Didemnum vexillum is a species of colonial tunicate in the family Didemnidae. It is commonly called sea vomit, marine vomit, pancake batter tunicate, or carpet sea squirt. It is thought to be native to Japan, but it has been reported as an invasive species in a number of places in Europe, North America and New Zealand. It is sometimes given the nickname "D. vex" because of the vexing way in which it dominates marine ecosystems when introduced into new locations; however, the species epithet vexillum actually derives from the Latin word for flag, and the species was so named because of the way colonies' long tendrils appear to wave in the water like a flag.

D. vexillum can grow on most hard substrates, including cobble, boulders, and artificial structures including boats, dock pilings, and aquaculture equipment. It can also grow over other organisms, including sessile invertebrates, algae, and eelgrass. Its appearance has given rise to the common names of sea vomit or marine vomit. It was first described from New Zealand as Didemnum vexillum by Patricia Kott in 2002. The species had been discovered previously in its native region of Japan.

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