Three new species of dactylogyrids (Monogenea) from the Araguaia River in Brazil are described herein. Specimens of Demidospermus ceccarellii n. sp., Demidospermus brachyplatystomae n. sp., and Demidospermus araguaiaensis n. sp. were collected from the gills of the siluriform fish, Brachyplatystoma filamentosum (Lichtenstein, 1819). The 3 new species can be easily distinguished from their congeners by the shape of the male copulatory organ (MCO) and accessory piece. Demidospermus ceccarellii n. sp. presents a J-shaped and robust MCO, a base with a short sclerotized margin, and a slightly curved and robust accessory piece with the base slightly dilated. In contrast, D. brachyplatystomae n. sp. has a thin MCO forming an incomplete ring, a base with an irregular sclerotized margin, and a long and curved accessory piece with a wider distal portion. Demidospermus araguaiaensis n. sp. has a thin MCO with a base directly coupled to the accessory piece and an articulated accessory piece that is sinuous in shape in the distal portion. In addition, the 3 species present haptoral hooks similar in shape and size and, thus, were compared in detail with the other Demidospermus species with this particular characteristic. This is the first record of monogeneans that are parasitic on B. filamentosum.