This research article published by Springer Nature Switzerland AG., 2020Lead-free perovskites have drawn much attention of researchers in the field of electronics and photovoltaics due to the toxicity issue of the lead halide perovskites. The methylammonium tin iodide CH3NH3SnI3 amongst others has become a viable alternative due to its eco-friendliness, as well as narrower bandgap and its wider visible absorption spectrum. In this study different theoretical approaches were employed in investigating the structural, electronic and thermodynamic properties of the orthorhombic phase (O-phase) of the CH3NH3SnI3 perovskite. By using the first-principle calculations with the density functional theory, a direct bandgap was determined at gamma symmetry points with three exchange–correlation functionals: PBE 1.12 eV, PBEsol 0.98 eV, and LDA 0.46 eV. Based on the comparison of lattice constants and bandgaps with the experimental values, the best performance resulted from PBE. The decomposition of the CH3NH3SnI3 perovskite into solid state products, CH3NH3I and SnI2, was considered; the enthalpy of the reaction ΔrH° (0 K) = 37 kJ mol−1 and enthalpy of formation of the O-phase perovskite ΔfH° (CH3NH3SnI3, 0 K) =  − 390 kJ mol−1 were evaluated, indicating the stability of the O-phase CH3NH3SnI3 at low temperature, in agreement with experimental findings.