The spin polarization of polarons in quasi-1D organic materials has been investigated by using the extended Su–Schrieffer–Heeger (SSH) model with spin-orbit coupling. Results show that the polaron is partly spin polarized, and that the electron–electron interaction and spin-orbit coupling compete with each other during the formation of spin polarization. The dependence of spin polarization on electron–phonon coupling is also revealed. Our results demonstrate that spin polarization is well correlated with polaron localization, thus providing useful guidance for exploring magnetic effects in organic materials.