The anatomy and development of the siluriform pectoral-fin spine is described, illustrated and a terminology is suggested for its parts. Catfish pectoral-fin spines exhibit considerable diversity of size, shape, robustness, surface texture and, especially, details of the dentated or serrated anterior and posterior margins. This study illustrates the variety, and taxonomic and phylogenetic significance of pectoral-fin spine diversity in the South American goliath catfishes of the tribe Brachyplatystomini, family Pimelodidae, based on examination of spines of post-juvenile and adult specimens representing all eight living species of Brachyplatystoma and Platynematichthys. Unique pectoral-spine characters and character combinations serve to distinguish all eight species. Features of the pectoral-spines that change with growth are also described. Within the current phylogenetic framework of Pimelodidae, brachyplatystomines show character-state transformations and synapomorphies of the pectoral-fin spines that support hypotheses of monophyly for the subgenera Malacobagrus (B. rousseauxii, B. filamentosum, B. capapretum), and Goslinia (new usage) (B. platynemum, B. juruense), and also suggest a close relationship between B. tigrinum and subgenus Goslinia. Platynematichthys notatus and B. vaillantii retain relatively plesiomorphic features of the pectoral-fin spines.