Thalasseus, the crested terns, is a genus of six species of seabirds in the tern family. The current genus name is derived from Greek Thalassa, "sea". It has a worldwide distribution, and many of its species are abundant and well-known birds in their ranges. This genus had originally been created by Friedrich Boie in 1822, but had been abandoned until a 2005 study confirmed the need for a separate genus for the crested terns.