The Parnaíba River is the largest hydrographic basin with its limits entirely within the northeastern region of Brazil. It is one of the few perennial rivers in the Brazilian Northeastern Region, running predominantly in the area of Caatinga on the eastern portion (with intermittent drainages associated to this type of environment), and partly extending into the Cerrado region on the western portion. The objective of this study was to describe the diversity of freshwater fishes of the Parnaíba River basin by conducting a comprehensive ichthyofauna inventory of the basin. The sampling design of this study was based on the placement of (1) Fixed Sites for seasonal collection (dry and wet season, two years) in the three portions of the main course of the Parnaíba River, and (2) ''Aquarap'' protocol collections which covered the entire basin. The collection effort consisted of 244 sampling sites surveyed along the entire basin between the years 2004 and 2011. Fish were collected with the use of seines, cast nets, gill nets and dip nets; only the first two fishing gears were used in Fixed Sites. As a result, 146 species of freshwater fish were recorded in the Parnaíba River basin, distributed in 103 genera, 36 families and 11 orders. The registered endemism reached a total of 54 species.