The goal of our work was to assess the ecotoxicological effects of plastic leachates from pristine and weathered items on a marine diatom. To do so, we exposed the marine diatom Phaeodactylum tricornutum to the leachates from pristine and UV-weathered self-reinforced (SR-) polylactic acid (PLA) and polypropylene (PP) items. As a positive control, we assessed the response of the diatom growth to two associated compounds (dedecan-1-ol and 2,4- di-tert-butylphenol), components of the tested items. Weathered plastic items were obtained after being exposed to 57-day artificial ultraviolet radiation, simulating 18-months of solar exposure in central Europe. Our results indicate that neither leachates from pristine nor weathered SR-PLA items had adverse effects on P. tricornutum growth. This outcome was corroborated by the measured concentrations of associated compounds (i.e. dodecan-1-ol) in leachates, which were at least three orders of magnitude lower than the 72-h EC50 of diatom growth (dodecan-1-ol: 1.56 mg / L, 2,4,-DTBP: 1.52 mg / L). Leachates from weathered SR-PP items at full-strength concentration induced 51% greater growth inhibition than control treatments, and 43% greater inhibition than leachates from pristine SR-PP items at the same concentration. Our results suggest that leachates from the tested weathered SR-PP items inhibited the population growth of P. tricornutum, although at plastic concentrations above environmentally relevant levels.