Didemnum is a genus of tunicates in the familyDidemnidae. Some species of Didemnum can be characterized as invasive species. This is particularly an issue off the east coast of the United States, where in early 2006, Didemnum were estimated to inhabit more than 175 square kilometers of this undersea region. Other areas where Didemnum are seen as invasive include Canada, New Zealand and the Netherlands.
Didemnum vexillum turned up on coast of state of Washington in 2009 and has been showing up in Coos Bay and Winchester Bay, Oregon as of 13 May 2010.