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Microplastics (MPs) are normally defined as small plastic wastes with a size of 1 μm to 5 mm in diameter. This tiny plastic debris is abundant in aquatic systems and poses a great threat to aquatic biota. To date, toxicological assessment of MPs is predominantly dependent on metazoan animals, although their applications are sometimes limited due to the high cost, narrow ecological niche, or ethical considerations. In this regard, unicellular eukaryotes (i.e., protozoa) that are ubiquitously present in nature represent a promising alternative for evaluating the toxicity of MPs. In this study, we selected <i>Paramecium bursaria</i> (<i>P. bursaria</i>) as a representative of protozoa and further investigated behavioral and molecular changes in MPs-exposed <i>P. bursaria</i>. Our results showed that following MPs uptake, <i>P. bursaria</i> exhibited various changes, including anomalies in swimming patterns, reduction in moving speed, impairment of avoidance behavior, elevation of oxidative stress, and potential disturbance of endosymbiosis. These elicited changes in <i>P. bursaria</i> in response to MPs exposure were pronounced and measurable. Overall, this study demonstrated that <i>P. bursaria</i> could serve as a promising alternative for the toxicological assessment of MPs and may be further applied to evaluate the toxicity of other environmental contaminants.