Olindias sambaquiensis, Muller 1861 (Hydrozoa: Limnomedusae: Olindiidae) is a neritic jellyfish endemic in the Blanca Bay area (South of Buenos Aires, Argentina). In this paper we describe the first report of a dermatitis caused by this jellyfish in a thirty-four-year-old marine biologist who presented with an erythematous vesicular eruption after contact with Olindias sambaquiensis. Some preliminary ultramicroscopic observations on the nematocysts of this jellyfish are also reported.