Abstract: With the increasing production and application of plastics, there is growing concern regarding the ubiquitous distribution of microplastics and their coexistence with various contaminants. Meanwhile, antibiotics have been ubiquitously detected in aquatic environments due to their extensive production, poor absorption, and improper disposal of unused drugs. Sulfamethoxazole (SMZ), commonly used in veterinary medicine and aquaculture as an antibacterial compound, has been found in aquatic environments frequently. Interestingly, SMZ is also used as a feed additive to promote animal growth in farms and fish aquaculture. Polyvinyl chloride (PVC) microplastics, one of the most commonly used plastic materials, have been detected in various environmental compartments, including sediment, surface water, marine, and freshwater. Despite the toxicity of PVC and SMZ has been extensively studied, the combined toxic effects induced by the two pollutants on aquatic organisms remain largely unknown. In this study, adsorption experiments between PVC and SMZ were conducted. Then, Daphnia magna, a commonly used aquatic crustacean in ecotoxicological assessments, was chosen as the model organism to investigate the joint toxic effects of PVC and SMZ through 48h acute and 21d chronic toxicity experiments. We assessed a chain of parameters, including survival, growth, reproduction, intestinal structure, and locomotor behavior. The results showed that PVC could absorb SMZ through physical interaction. In the acute toxicity experiments, the presence of PVC (1 mg/L) reduced the SMZ content in D. magna individuals, thereby attenuating the lethal effects induced by SMZ exposure. In the chronic toxicity experiments, exposure by SMZ alone or in combination with PVC (1 mg/L) showed no significant effect on the mortality ratio, number of molts, or body length of the D. magna. However, the exposure to individuals of D. magna by SMZ alone or in combination with PVC aroused oxidative stress, intestinal damage, and locomotor inhibition. Moreover, the presence of PVC exacerbated the toxic effects. In conclusion, the findings revealed both antagonistic and synergistic toxic effects of PVC on SMZ, depending on the exposure dose and time. The chronic assays (21d) performed with the mixture of environmentally relevant concentrations of contaminants indicated that PVC enhanced the toxicity of SMZ to D. magna. This study provides new insights into the ecotoxicological effects caused by the co-existence of PVC and SMZ in aquatic environments. The study can also convince a reference for the underlying mechanisms of environmental behavior and toxicity aroused by the coexistence of microplastics and antibiotics on aquatic organisms.