In this study, the electronic and spin properties of SnX (X = P, As, Sb, Bi) monolayers under biaxial strain were investigated using density functional theory. The electronic and spin properties were studied for two structures of SnX with mirror symmetry and inversion symmetry. We compared the band structure of SnX with and without spin–orbit coupling to determine the importance of spin–orbit interaction for its band structure. The locations of the valleys in the conduction and valence bands were explored for various strains. We found that the bandgap was modified by considering spin–orbit coupling in the presence of strain. In addition, we calculated spin-splitting in the conduction and valence band in the presence of strain for the structure with mirror symmetry. The projected densities of states and band structures clarified the origins of these large spin-splittings. The lack of inversion symmetry and mirror symmetry induced the persistent spin helix. The corresponding spin–orbit strength was explored for compounds at various biaxial strains.