Single crystals of ammonium tetroxalate dihydrate doped with p-nitrophenol (PNP doped ATOXAL) were grown using the slow evaporation solution growth technique. The harvested crystals possessed the P1¯ space group and occupied the triclinic crystal system. From UV–vis-NIR spectrum the optical parameters like optical band gap (Eg), refractive index (n), electric susceptibility (χc) and real and imaginary parts of the dielectric constant were found to be 3.276 eV, 1.58, 0.199, 2.498 and 0.93 × 10−5 respectively at the lower cutoff wavelength. The refractive index value calculated using Abbe's refractometer was 1.581. The crystal exhibited negative photoconductivity. The photoluminescent emission spectrum was recorded and studied. The mechanical properties have been studied using Vicker's microhardness tester and the material belongs to the soft category. The thermal property of the crystal was analysed from the TGA/DTG/DSC curves. The dielectric studies carried out for varied frequencies and temperatures reveal that the material exhibits non-Debye β relaxation. Z-scan studies show that the grown crystal has negative nonlinearity and saturation absorption. © 2018 Elsevier Ltd