Results are presented on species richness and distribution of the crustacean fauna obtained by iXingu Project expeditions along the middle and lower Xingu River during low and high water seasons primarily from 2013–2014, prior to the completion of the Belo Monte dam complex. The checklist includes the parasitic and decapod crustacean species surveyed in the Xingu River from the mouth of its tributary, the Iriri River, to its confluence with the Amazon River. With the addition of records from the scientific literature, a total of 32 species representing 18 genera in eight families are known to occur in the middle to lower Xingu basin: four species of fish lice, family Argulidae, four parasitic isopods, families Corallanidae (1 species) and Cymothoidae (3), 13 decapod crabs, families Pseudothelphusidae (5) and Trichodactylidae (8), and 11 decapod shrimps, families Sergestidae (1), Euryrhynchidae (3), and Palaemonidae (7). Seventeen species are recorded for the first time from the Xingu basin, highlighting the importance of focused sampling of decapods and other crustaceans in major sub-basins of the Amazon. The lower Xingu, below the Volta Grande rapids, held the most diversity with 19 species. The Volta Grande rapids (between Altamira and the lower Xingu) and the middle Xingu above Altamira exhibited 17 and 15 species, respectively. Based on comparisons to other Amazon and South American river basins, the middle and lower Xingu River supports a remarkably high diversity of decapods.