A new species of the Dactylogyridae is reported parasitizing the gills of Oxidoras niger (Valenciennes, 1821) from the Peruvian Amazon. Cosmetocleithrum gigas sp. n. does not show any resemblance to other species of the genus and can be morphologically differentiated from other congeneric species by presenting a hammer-shaped accessory piece; a small haptor with small bars and anchors; both anchors presenting developed superficial roots and reduced deep roots; conspicuous folds on both roots and bars hat-shaped, expanded laterally forming two protuberances covered with rough folds. Additionally, C. gigas sp. n. is the largest species of the genus so far reported.