The widespread use of titanium dioxide nanoparticles (TiO2 NPs) in many applications has led to its significant release to aquatic systems. Hence, the present study was performed to evaluate effects of TiO2 NPs on crayfish Procambarus clarkii, which is often used as a bioindicator of water pollution. Adult male specimens were treated with 25, 125, and 250 mg/l of TiO2 NPs for 28 days. Mortalities were 0, 3.3, and 10% for animals treated with 25, 125, and 250 mg/l of TiO2 NPs, respectively. The level of titanium bioaccumulation in different tissues was as follows, gills > hepatopancreas > green glands > muscles. TiO2 NPs-exposed crayfish showed significant increase in the levels of catalase, glutathione-S-transferase, glutathione peroxidase, gamma-glutamyl transferase, glutathione, and metallothioneins in the hepatopancreas. Exposure of the crayfish to 250 mg/l TiO2 NPs caused severe histopathological alterations. In the hepatopancreas, the most prominent pathological changes were tubular disruption and inflammatory infiltration. Under transmission electron microscope (TEM), ruptured microvilli, deformed mitochondria and fragmentation of rough endoplasmic reticulum (RER) were observed. In the gills, swelling of gill lamellae, disorganization and degeneration of epithelial cells were noted under the light microscope (LM). Under electron microscope (EM), gills of crayfish treated with 250 mg/l TiO2 NPs showed vacuolation, dense granules, and diminution in the number of apical plasma membrane infoldings. In conclusion, TiO2 NPs caused alteration in antioxidant activities and severe histopathological changes.