The hydromedusa Gonionemus vertens A. Agassiz, 1862 was first detected in the Berre Lagoon, south of France, in July 2016. Early June 2017, following an early rise in temperature, much higher numbers of specimens were reported leading to several beach goers being stung and one going into anaphylactic shock. Effort to rehabilitate this highly anthropogenically altered lagoon has led to the redevelopment of the Zostera meadows, which might be favoring the clinging jellyfish. Developmental aberrations of specimens were noted, which could be linked to the remaining presence of chemicals from the industrial development period. Impact of this predator species on the functioning of the lagoon ecosystem should also be studied.