Microplastics can interact with contaminants in the surrounding environment, and this potential carrier effect of microplastics for contaminants raises global concern. However, there are still knowledge gaps surrounding the question of how the interactions between microplastics and contaminants affect the bioaccumulation and ecotoxicological effects on aquatic organisms. In the present study, both pristine and environmentally aged microplastics were employed in vitro and in vivo studies to investigate the effects of microplastics on the bioaccessibility and bioavailability of antibiotics in marine fish. The results showed that aged microplastics increased the adsorption and desorption of antibiotics in simulated fish digestion systems. In addition, smaller microplastics increased antibiotic bioaccessibility during fish digestion compared with larger microplastics. The interaction between microplastics and antibiotics modulated the bioavailability of antibiotics in the fish under coexposure scenarios, and antibiotic-loaded microplastics increased the bioaccumulation of antibiotics in the fish gut. These findings revealed that aged and precontaminated microplastics could act as pollutant carriers that may pose ecological risks to aquatic ecosystems.