Gas-phase electron diffraction was applied for the molecular structure determination of octamethylporphyrin tin(II), SnN₄C₂₈H₂₈, at the temperature of 706(10) K. The molecule was found to possess C_4v symmetry with the Sn atom 1.025(30) Å above the plane of the N atoms and the following main internuclear distances (r_h1, Å): Sn–N = 2.301(9), C_α–N = 1.360(8), C_α–C_β = 1.453(4), C_α–C_m = 1.395(4), C_β–C_CH3 = 1.498(4). Quantum chemical calculations, DFT (B3LYP, BP86, PBE, PBE0) with cc-pVDZ, cc-pVTZ and cc-pVQZ basis sets reproduce the experimental bond distances with accuracy within 0.03 Å. According to NBO(B3LYP/cc-pVTZ) analysis, the direct donation gives a prevailing contribution to Sn–N bonding, decreasing the net charge on Sn from formal +2 to +1.28. The substitution effects at the pyrrole rings are discussed. The ability of different theoretical methods to predict the structure of this compound is analyzed.