Plastic has become an indispensable material in everyday human life and its mass production continues to increase because of its softness, transparency and economic viability. Due to increasing production and use, improper plastic disposal and fragmentation into smaller particles such as microplastics (MPs) pose a serious problem because of their potential intake by plants and animals, ultimately affecting humans. In this study, the effects of MPs from two types of polymers, polystyrene (PS-MPs) and polymethyl methacrylate (PMMA-MPs), on the occurrence of oxidative stress in duckweed (Lemna minor) were investigated. The content of hydrogen peroxide (H2O2), lipid and protein damage as well as changes in the activities of antioxidant enzymes and the content of the non-enzymatic antioxidant proline were analyzed. The H2O2 content increased in the treatment with the lowest PS-MPs concentration, while other treatments showed no significant changes compared to the control. No significant lipid and protein damage or changes in the activities of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT) were recorded in any of the treatments studied. Although duckweed exposure to MPs did not result in increased activity of ascorbate (APX) and pyrogallol peroxidase (PPX) compared to the control, higher concentrations of PS-MPs treatment showed a significant increase compared to PMMA-MPs. On the other hand, increased proline content at the highest examined concentration was only observed in PMMA-MPs. The obtained results indicate that both types of MPs activate the antioxidant system of duckweed, with PS-MPs a stronger enzyme activation, while PMMA-MPs have a greater impact on non-enzymatic antioxidant proline.