Museum collections continue to play a critical role in taxonomic research by preserving historical material and biodiversity records that might otherwise have been lost. In the present study, material from the Iziko South African Museum, Cape Town, was used to morphologically redescribe Gnathia spongicola Barnard, 1920, as well as describe two new species, Gnathia breviculasp. nov. and Gnathia lanciferasp. nov., from the Temperate Southern Africa (TSA) marine realm. The triangular anterior margin of pereonite 6, with its midpoint projecting forward to reach pereonite 4 and bisect pereonite 5, is a feature rarely recorded among Gnathiidae (previously noted only in Gnathia disjuncta). However, this character is present in all three species examined in the present study. Of the three, Gnathia spongicola is characterised by a weak and bifid mediofrontal process; a single superior frontolateral process; strong and equally apically bifid, pronounced and pointed supraocular lobes; as well as strong distally curved mandibles with dentated blades. Gnathia breviculasp. nov. is distinguished by two strong, rounded and extended frontolateral processes; broadly rounded and minimally developed supraocular lobes; and weakly curved mandibles with slight distal curvature. Gnathia lanciferasp. nov. can be characterised by a mediofrontal process that is weakly rounded; two strong and conical superior frontolateral processes; rounded and pronounced supraocular lobes; and strong, distally curved, crescent-shaped mandibles with dentation. These descriptions increase the number of known species of Gnathiidae from the TSA marine realm to 11.
