The unexpectedly high affinity of microplastics (MPs) for heavy metals has attracted increasing attention in recent years. However, there is a scarcity of information on the effects of coexisting constituents on plastic–metal interactions, especially in terms of metal speciation. This study aimed to explore the role of benzophenone-type UV filters on the sorption, oxidation state, and toxicity of chromium (Cr) by pristine and UV-irradiated polystyrene MPs (PSMPs). Results showed that Cr uptake by PSMPs was remarkably higher when UV filters were present. This enhanced affinity was attributed to the formation of Cr–UV filter complexes together with multilayer sorption on PSMPs’ surfaces. Subsequent chemical state measurements indicated alteration of the oxidation state of MP-bound metals as a higher Cr­(VI) abundance was identified on the PSMPs with coexisting UV filters. More importantly, our toxicity assessments found that PSMP-bound Cr with a higher oxidation state displayed more severely inhibited microalgae growth. It is noteworthy that toxicological effects of Cr–UV filter complexes were observed only with PSMPs, which thus reaffirms the unique role of MPs in accumulating and transforming contaminants in natural environments. Overall, this study provides the first evidence into the effects of coexisting constituents on the oxidation states and toxicities of MP-bound heavy metals.