Brachyplatystoma capapretum n. sp., a pimelodid catfish from the Amazon River, is described and diagnosed by its fine, densely spaced outer jaw teeth and relatively few rows of straight, depressible inner teeth; juveniles with large dark blotches on and above lateral line; adults with exceptionally dark pigmentation dorsally, abruptly countershaded near lateral line; broad, shallowly forked caudal fin. The systematic position of the new species was investigated with a maximum parsimony phylogenetic analysis of morphological characters. The resulting cladograms identify a nested set of unambiguous characters supporting the following monophyletic groups and taxonomic recommendations. Brachyplatystomatini new tribe is established for Brachyplatystoma and Platynematichthys and supported by two synapormorphies: two-chambered gas bladder connected by paired lateral tubes; cleithrum with blade-like ventral crest between pectoral spine articulation and posterior process of cleithrum. The monotypic genera Goslinia and Merodontotus are placed in synonymy with Brachyplatystoma characterized here by two synapomorphies: suspensorium greatly expanded mediodorsally, forming a massive plate approaching parasphenoid; juveniles with greatly elongated caudal-fin filaments developed from single outermost, unbranched principal rays. Bleeker's genus group name Malacobagrus is employed as a subgenus within Brachyplatystoma to include B. filamentosum, B. capapretum, and B. rousseauxii. Monophyly of Malacobagrus is supported by five synapomorphies: adults developing extensive reticulated system of lateralis tubules on body; Weberian complex with thick, ventral bony platform at gas bladder attachment; subquadrangular shape of opercle; pectoral spine with sulcus along anterior edge; small, non-sutured endopterygoid. Two unique synapomorphies place B. capapretum and B. filamentosum together as sister species: juveniles with spots on and above lateral line; caudal peduncle subcircular in cross-section.