Microplastics are ubiquitous in aquatic ecosystems and may affect organisms through interactions with coexisting contaminants. Their large surface area enables the adsorption of environmental pollutants and toxins, potentially altering their transport and bioavailability. However, the ecological consequences of such vector-mediated exposure remain poorly understood. Microcystin-LR (MC-LR), a prevalent cyanotoxin produced during harmful cyanobacterial blooms, can persist in aquatic environments and associate with microplastics, potentially creating an alternative pathway of biological exposure. This study investigated whether polystyrene (PS) microplastics act as vectors of MC-LR and alter its ecotoxicity in Daphnia magna. Four exposure scenarios were established to evaluate the role of PS in mediating MC-LR exposure and toxicity: control, PS alone, freely dissolved MC-LR, and MC-LR–adsorbed PS. Chronic toxicity was evaluated over 21 days by integrating physiological, biochemical, and molecular endpoints. Among the treatments, MC-LR–adsorbed PS induced the most pronounced biological responses. Survival decreased to 40%, compared with 70% in the PS treatment and 95% in the freely dissolved MC-LR treatment, and reproductive output was significantly reduced. Exposure to MC-LR–adsorbed PS resulted in a significant increase in lactate dehydrogenase activity and broad transcriptional activation of genes involved in antioxidant defense, xenobiotic metabolism, cellular efflux, and oxygen transport. Freely dissolved MC-LR induced moderate increases in antioxidant and detoxification enzyme activities without marked physiological impairment. Gut-recovered MC-LR–adsorbed PS exhibited surface-associated transformations during intestinal passage. These findings demonstrate that PS microplastics can act as vectors of MC-LR, enhancing ingestion-mediated exposure and ecological risks in aquatic ecosystems.