A new hydromedusa species of the genus Bougainvillia (Anthoathecata; Bougainvillidae) is described. Bougainvillia pagesi sp. nov. was collected in shallow coastal waters from southern Brazil (~25–26°S) and northern Argentina (~36–38°S). It differs from other species of the genus mainly by two characters: (i) relatively large size (up to ~20 mm wide and high) and (ii) voluminous folded hanging gonads at perradial walls of the manubrium and extending to the proximal region of the radial canals. Its cnidoma is similar to the other species of the genus, with desmonemes (undischarged capsules: 7.08–10.01 × 2.5–5.59 μm) at marginal tentacles and microbasic euryteles (undischarged capsules: 7.66–11.44 × 2.64–4.43 μm) and smaller (undischarged capsules <2 μm) desmonemes at oral tentacles.