Microplastics (MP) widely distributed in aquatic environments have adverse effects on aquatic organisms. Currently, the impact of MP on toxigenic red tide microalgae is poorly understood. In this study, the strain of Alexandrium pacificum ATHK, typically producing paralytic shellfish toxins (PST), was selected as the target. Effects of 1 and 0.1 μm polystyrene MP with three concentration gradients (5 mg L⁻¹, 25 mg L⁻¹ and 100 mg L⁻¹) on the growth, chlorophyll a (Chl a), photosynthetic activity (F_v/F_m) and PST production of ATHK were explored. Results showed that the high concentration (100 mg L⁻¹) of 1 μm and 0.1 μm MP significantly inhibited the growth of ATHK, and the inhibition depended on the size and concentration of MP. Contents of Chl a showed an increase with various degrees after MP exposure in all cases. The photosynthesis indicator F_v/F_m of ATHK was significantly inhibited in the first 11 days, then gradually returned to the level of control group at day 13, and finally was gradually inhibited in the 1 μm MP treatments, and promotion or inhibition to some degree also occurred at different periods after exposure to 0.1 μm MP. Overall, both particle sizes of MP at 5 and 25 mg L⁻¹ had no significant effect on cell toxin quota, and the high concentration 100 mg L⁻¹ significantly promoted the PST biosynthesis on the day 7, 11 and 15. No significant difference occurred in the cell toxin quota and the total toxin content in all treatments at the end of the experiment (day 21). All MP treatments did not change the toxin profiles of ATHK, nor did the relative molar percentage of main PST components. The growth of ATHK, Chl a content, F_v/F_m and toxin production were not affected by MP shading. This is the first report on the effects of MP on the PST-producing microalgae, which will improve the understanding of the adverse impact of MP on the growth and toxin production of A. pacificum.