Semi organic nonlinear optical crystal of guanidinium manganese sulphate hydrate (GuMnS) was grown from its aqueous solution by slow evaporation solution growth technique. Formation of the crystalline compound was confirmed by powder X-ray diffraction analysis (PXRD).The compound crystallizes in the triclinic crystal system with space group Pī. The functional groups present in the crystal have been identified by FTIR spectroscopic analysis. UV–Vis–NIR spectral study indicates that the grown crystal is transparent in the entire visible region with a lower cut off wavelength 229 nm and the band gap value is found to be 5.06 eV. Nonlinear refractive index (n2), absorption coefficient (β) and third order nonlinear susceptibility (χ(3)) were determined using Z-scan technique. Thermo gravimetric and differential thermo gravimetric analyses (TG-DTG) were performed to analyse the thermal behaviour of the grown crystal. The dielectric constant and dielectric loss were studied as a function of frequency of the applied field. The ferroelectric nature of the grown crystal was analysed by P–E hysteresis loop. The mechanical behaviour of the grown crystal was studied by Vickers’s micro hardness test. The laser induced surface damage threshold (LDT) of the grown crystal was found to be 1.157 GW/cm2. The obtained results show that guanidinium manganese sulphate crystals are potential materials in NLO device applications. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.