We herein report the Leptomedusae Eutonina scintillans (Bigelow, 1909) for the Buenos Aires coast (38°S) as a consequence of a research project dealing with hydromedusae distributions in the southwestern Atlantic Ocean (34°-55°S). This is the first record of this species, which was previously known for tropical waters (Brazil, 16°S), in the temperate waters of the South American Atlantic Ocean. Possible reasons for its appearance so far south are discussed