A comprehensive description of photo-chemical processes in materials, comprising spin-forbidden processes such as intersystem crossing and phosphorescence, implies taking into account spin–orbit coupling. We present an efficient implementation of a perturbative spin–orbit coupling correction for the Tamm–Dancoff approximation of linear-response time-dependent density functional theory within a mixed Gaussian and plane wave framework relying on spin–orbit coupling corrected pseudopotentials. The implementation is validated for a benchmark set of small aromatic molecules, with mean errors in excitation energies and spin–orbit coupling matrix elements being in the range of 0.1–0.6 eV and 1.0–14.4 cm−1, respectively, in comparison with density functional theory and density functional theory multi-reference configuration interaction reference results. Computational timings are given for a bismuth-containing metal–organic framework.