A new species of ergasilid copepod, Ergasilus xinguensis n. sp., is described from females found on the gills of two cichlid fishes, Geophagus argyrostictus (Kullander, 1991) (type host) and G. altifrons (Heckel, 1840), from the Brazilian Amazon. The new species can be distinguished from congeners by the unique combination of the following characteristics: the cephalothorax is not inflated and is slightly constricted, the first antennulary segment bears 3 setae, maxillule with 3 unequal outer setae without minute process medially, maxilla has a large syncoxa with one seta near its basis, first and fourth legs are with a 3-segmented endopod, base of the exopod in leg 2 with a conspicuous bluntly-pointed projection and caudal ramus with two rows of curved conical spinules on ventral surface. The new species is the second member of Ergasilus von Nordmann, 1832 found on cichlids of the genus Geophagus (Heckel).