Herein, we report a simple fluoride-mediated reaction for the direct mono- and dialkylthio-functionalization of unsubstituted perylenediimides (PDIs) under very mild conditions. The aromatic substitution reaction offers the possibility to introduce primary, secondary and, even, tertiary alkanethiols either on the 1- or on the 1,6-bay positions of unsubstituted PDIs. 1,6-DialkylthioPDIs show that absorption and fluorescence spectra shifted to the red when compared with the unsubstituted PDI, with Stokes shifts around 70–80 nm.