Microplastics (MPs) have emerged as contaminants of concern as a consequence of massive increases in plastic production over the past 50 years. MPs have been detected in all ecological habitats, but aquatic habitats are the most affected. MPs can cause direct toxicity to aquatic organisms and can also act as vectors that increase the bioavailability of other pollutants. Cadmium (Cd2+) is a widespread and highly toxic metal with a high chance of co-existing with MPs in aquatic environments. The aim of this study was to evaluate the direct and indirect toxic effects of MPs by single and combined exposure of MPs and Cd in zebrafish. High-density polyethylene pellets, either virgin or naturally aged, were ground to micro-size particles. The first part of this study measured the Cd adsorption capacity of virgin and aged MPs. The aim of the second experiment was to measure the effects of the two types of MP, alone, pretreated with Cd, or in combination with ionic Cd, on the bioaccumulation of Cd, levels of metallothionein, behavior, and histopathology of adult Zebrafish (Danio rerio). The adsorption capacity (qe) experiment was done by incubating MPs in Cd solutions and measuring Cd decrease from solution at different time intervals. This experiment showed that aged MPs had higher adsorption capacity than virgin MPs which is most likely to be caused by the change in MPs physiochemical properties. In the second experiment, zebrafish were exposed to an environmentally realistic concentration of MPs (0.1% MPs enriched diets) and Cd (50 µg/L) and a combination of the two for 21 days. Fish mortalities were recorded due to waterborne Cd2+ exposure while MPs had no effect on survival. Fish growth and development were not affected. Compared to a waterborne Cd alone, exposure to MPs and waterborne Cd resulted in higher Cd bioaccumulation (specifically in males) while MT level was not affected. There was a minimal transfer of Cd from pre-treated MPs. Anxiety-like behavior increased significantly after co-exposure to MPs and waterborne Cd. The zebrafishes social tendency decreased slightly in all treatments but only dietary exposure to aged MPs showed a statistically significant effect. Slight impacts on fish boldness and shoaling behavior were observed. Gills and liver histopathology showed slight alteration while most histopathological lesions were in the intestine. Histopathological results indicated the capability of Cd to desorb out MPs and cause toxicity. Overall, the study supports the hypothesis that MPs can act as vectors for Cd. Interestingly, MPs increased the uptake of waterborne Cd by zebrafish but did not appear to be direct vectors, suggesting they acted by an indirect mechanism. Noteworthy, pre-treated MPs had no distinct impacts on zebrafish mortality, Cd uptake, and behavior but at the histopathological level, they had higher impacts. This study demonstrates that MPs can enhance the bioaccumulation of Cd, but further study is needed to elucidate the mechanism.