Microplastics are ubiquitous pollutants in the aquatic environment. However, our understanding of the interaction of chemicals, particularly synthetic pyrethroids, with microplastics and the potential toxic effects of sorbed contaminants on aquatic organisms under realistic conditions is still extremely limited. In this study, we examined whether the presence of polyethylene (PE) microplastics can affect the acute toxicity of the synthetic pyrethroid bifenthrin to an invertebrate Chironomus tepperi in both synthetic and river water. Bifenthrin alone was, as expected, acutely toxic to exposed larvae (LC 50 of 0.5 μg/L after 48 h exposure). The addition of microplastics to synthetic water significantly reduced the toxicity of bifenthrin (apparent LC 50 = 1.3 μg/L), most likely because sorption of bifenthrin to microplastics reduced its bioavailability to the exposed larvae. A sorption capacity experiment showed that >92% of bifenthrin was sorbed to microplastics. In river water containing 9.6 mg/L organic carbon, bifenthrin alone was less toxic (LC 50 = 1.3 μg/L) than in synthetic water. Strikingly, the addition of microplastics to river water did not mitigate bifenthrin toxicity (apparent LC 50 = 1.4 μg/L), most likely due to greater interaction of bifenthrin with organic carbon than with microplastics. While PE microplastics reduced the negative effects of bifenthrin in synthetic water, the presence of organic carbon in river water without microplastics also reduced toxicity. This suggests that while sorption of contaminants to microplastics does occur, it may not be as relevant under environmentally realistic conditions with mg/L concentrations of organic matter.