A new species of Tatia is described from the lower rio Iguaçu basin, Paraná State, Brazil. The new species is distinguishable from congeners by having large, irregular pale blotches over dark brown base coloration, absent longitudinal stripe, caudal fin with round to striate dark brown spots over light base in adults or entirely dark in juveniles, and by having a long humeral process. Sexual dimorphism of the new species is marked by differences among genital and urinary apertures, size proportions between upper and lower lobe of caudal fin, and size, width and presence of antrorse and retrorse spines on anal-fin rays.