A new species of Monogenea - Trinibaculum pinctiarum n. sp. is described from the gills of Astyanax fasciatus known as "lambari do rabo vermelho", sampled in the Paranapanema River, Jurumirim reservoir, São Paulo State, Brazil. The new species is allocated to the genus Trinibaculum because it has a double ventral bar and dextro-lateral vagina. It is the second species (first species is Trinibaculum altiparanae) belonging to this genus that parasitizes the gills of the fish genus Astyanax. This monogenean is similar to T. altiparanae regarding the haptor structures and the bulbous distal vagina. However, Trinibaculum pinctiarum n. sp. differs in having: (1) coiled male copulatory organ, (2) accessory structure shaped as tweezers, (3) absence of postmedian process in the ventral bar, and (4) hooks with depressed thumb. The phylogenetic hypothesis of the genus Trinibaculum (Consistency ind...