Four previously unknown species of hydromedusae are described from plankton samples collected during oceanographic cruises undertaken along the south‐western coast of Africa. The Anthomedusa Bythotiara capensis sp.n., known from a single specimen, was found in shallow waters of the Cape region (South Africa). The Leptomedusa Margalefia intermedia gen. et sp.n. collected in Namibian waters, possesses desmoneme cnidocysts. This category of cnidocysts is previously unreported in the Leptomedusae, but is common in Anthomedusae and in the limnomedusan family Proboscidactylidae. Margalefia seems to occupy a position intermediate between the families Tirannidae and Laodiceidae. Two species of Limnomedusae, Aglauropsis edwardsii sp.n. and Proboscidactyla menoni sp.n. were frequent at several stations in Namibian waters between depths of 50 m and the surface.