This study aimed to investigate how an ultralow content of a molybdenum disulfide (MoS2) two-dimensional particle affects the photodegradation mechanism of polystyrene (PS). Here, an accelerated weathering study was presented on neat polystyrene and its nanocomposites produced with 0.001, 0.002, 0.003 and 0.005 wt% of molybdenum disulfide (MoS2) exposed for various irradiation intervals (up to 8 weeks). The polymer photo-transformations were monitored using size exclusion chromatography (SEC), infrared spectroscopy (FTIR), and UV-Vis spectroscopy. The FTIR and UV/Vis results indicate that the PS degradation mechanism was not altered by the presence of MoS2particles; however, the degradation reactions were slowed down at higher MoS2contents (>0.003%). The SEC results proved the stabilizer effect due to MoS2particles, whereM¯n,M¯w, andM¯w/M¯nvalues after 8 weeks were less modified when compared with the neat PS results. The MoS2acted as a UV stabilizer, and these two-dimensional particles acted by deactivating the free radicals generated by the PS matrix, even considering the low amount of the filler (<0.005 wt%).