Didemnum is a genus of colonial tunicates in the family Didemnidae.

[1][2] Species in this genus often have small calcareous spicules embedded in the tunic and form irregular or lobed colonies.

[3] In early 2006, Didemnum vexillum was found covering a 230 km2 area of cobble habitat in Georges Bank off the coast of New England,[4] and is classified as an invasive species of greatest concern in coastal areas throughout Europe,[5] New Zealand, and North America.

[6] Species in this genus can be found in tropical or temperate regions.

Some tropical species such as Didemnum molle have photosynthetic algae in their tunics.