Organic solid-state lasers (OSSLs) based on singlet fluorescence have merited intensive study as an important class of light sources. Although the use of triplet phosphors has led to 100% internal quantum efficiency in organic light emitting diodes (OLEDs), stumbling blocks in triplet lasing include generally forbidden intersystem crossing (ISC) and low quantum yield of phosphorescence (ΦP). Here, we reported the first triplet-phosphorescence OSSL from nanowire microcavity of a sulfide-substituted difluoroboron compound. As compared with the un-substituted parent compound, the lone pair of electrons of sulfur substitution plus the intramolecular charge transfer interaction introduced by the nitro moiety lead to an highly efficient T1 (π,π*)←S1 (n,π*) ISC (ΦISC = 100%) and a moderate ΦP (10%). This, plus the optical feedback provided by nanowire Fabry-Perot microcavity, enables triplet-phosphorescence OSSL emission at 650 nm under pulsed excitation. Our results open the door for a whole new class of laser materials based on previously untapped triplet phosphors.