The present study aims to assess the induced nanotoxicity of arsenic nanoparticles (AsNPs) on different organs of fresh water fish Labeo rohita. AsNPs were synthesized by chemical reduction method using sodium arsenite as precursor, ice-cold sodium borohydride as reducing agent, and sodium hydroxide to adjust the solution pH. The synthesized AsNPs were characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) for optical, structural, and morphological investigations. The UV-Vis absorption peaks occurring at around 300 nm indicated the presence of AsNPs in colloidal sample. The rhombohedral crystalline nature and metallic purity of AsNPs with crystallite size of 30 ± 1 nm were confirmed by characteristic peaks of XRD pattern. The SEM micrograph revealed the almost spherical shape and 40 ± 10 nm average size prepared AsNPs. For assessment of induced nanotoxicity, juveniles of Labeo rohita (L. rohita) were exposed to three different concentrations of AsNPs (namely, 1, 10, and 20 mg/L) for 30 days (n = 15 per group), and the control fish was kept untreated. It was observed that the routine behavior activities (such as swimming, mutual interactions, and feed intake) were affected by AsNPs. The growth of AsNPs treated fish was found retarded as compared to the control fish. Total erythrocyte count, total leukocyte count, and hemoglobin and hematocrit values were low in the AsNPs treated fish. Immunobiochemical assays revealed that protein level was altered in the AsNPs treated fish. The levels of antioxidant enzymes catalase and superoxide dismutase were low in the treated fish. The histological alteration induced by AsNPs in liver, gills, and kidneys demonstrated the damage in form of glomerulus shrinkage, vacuolation, inflammation, necrosis, lamellar disorganization, and hemorrhage in comparison with untreated fish. The results of the present study indicate that AsNPs exposure causes behavior, growth, hematology, immunobiochemical, and histological shortcomings in L. rohita.