Microplastics (MPs) are plastic particles smaller than 5 mm that accumulate in ecosystems and can cause toxicity in organisms by affecting multiple biological processes. This study investigates the effects of poly(methyl methacrylate) microplastic microspheres (MPs, 200 µm diameter) on Caenorhabditis elegans, a widely used model in ecotoxicology. Nematodes were exposed to MPs at concentrations of 0.01, 0.1, 1, and 10 mg/mL, and various toxicological endpoints were assessed. The uptake of MPs was evaluated by µFT-IR analysis. The results indicate that MPs induce a concentration-dependent reduction in body length and alterations in the reproduction rate. Lifespan was also significantly reduced, with a 20% decrease at the highest concentration. Intestinal permeability assays revealed disruption of gut integrity at higher concentrations, and oxidative stress analysis showed a 1.8-fold increase in reactive oxygen species (ROS) levels at 10 mg/mL. Gene expression analysis via real-time qPCR indicated the upregulation of genes involved in oxidative stress and in DNA repair mechanisms. Additionally, the longevity-related transcription factors daf-16 and skn-1 were modulated, suggesting an adaptive stress response. These findings suggest that MPs impair growth, reproduction, and oxidative stress response in C. elegans, emphasizing the potential risks associated with microplastic exposure.