Two neogastropod species occur in brackish intervals in the Pebas Formation (late Middle to early Late Miocene) of Peru and Colombia in western Amazonia. Purpura woodwardiRoxo, 1924, is assigned to MelongenaSchumacher, 1817 (Melongenidae), and ?Nassarius reductus (Nassariidae) is recognized as a new species. These gastropods are among the very few marine invaders in the otherwise freshwater Pebas fauna. The small number of marine to freshwater transitions among South American molluscs contrasts with the situation among South American fishes and southeast Asian molluscs. It may be related to seasonal fluctuations in water level and anoxia in present-day South American freshwater environments, as well as to predation and productivity.