This study defines a new bivalve species, Naiadites devonicus nov. sp., of Famennian (Late Devonian) age. The material was excavated from the non-marine deposits of the Waterloo Farm Lagerstätte in South Africa, which form part of the Witpoort Formation (Witteberg Group, Cape Supergroup) of the Cape Fold Belt region. The type material of Naiadites devonicus nov. sp. is preserved in metamorphosed, clayey to silty mudstones of a coastal estuarine to lagoonal facies, deposited along the shoreline of the Agulhas Sea, in a high-palaeolatitude Gondwanan setting. The new bivalve species is the stratigraphically oldest representative of the genus Naiadites Dawson. It is a faunal element of a high-latitude palaeoecosystem, immediately preceding the Hangenberg extinction event at the end of the Late Devonian.