Gonionemus vertens, the clinging jellyfish, is a small species of hydrozoan in the family Olindiidae found in coastal regions throughout large parts of the Northern Hemisphere.

[2][3] Gonionemus vertens may catch prey when attached to surfaces during the day, but they mostly feed at night in open water,[3][4] eating zooplanktonic organisms like fish larvae and tiny crustaceans.

[2] Some authorities also include areas south to Vietnam in its native range,[3] and the northeast Pacific from the Aleutian Islands to Puget Sound (the type locality),[4] while others consider these as non-native introductions.

[2][3][5][9] In much of its range, Gonionemus vertens has a strong sting, which can cause pain, a swollen throat, chest tightness, muscle cramps, neuropsychiatric changes and anaphylactic shock.

[5] In the past, it was also regarded as harmless on the Atlantic coast of North America and the recent strong stings in this region possibly is the result of introductions of the Asian form of the species.