Concerns about microplastic pollution have risen as numerous studies have reported detection of microplastics in foods, including seafood. One emerging concern is the ability of microplastics to vector pathogens that can adhere to biofilms on microplastic surfaces. Here, we investigated whether microplastics can facilitate zoonotic protozoan parasite contamination in shellfish. Oysters were selected for this study because they are commonly eaten raw and can harbor zoonotic protozoan pathogens. Acclimated live oysters were exposed in closed aquaria to Cryptosporidium, Giardia, and Toxoplasma (oo)cysts that had been incubated in seawater either as protozoa alone (P treatment) or with preconditioned polyester microfibers (P + M treatment). After overnight exposure, oysters were transferred to clean seawater flow-through aquaria for depuration. Over the experimental period, oysters exposed to both protozoa and microfibers had significantly higher numbers of protozoan pathogens than oysters exposed to protozoa alone. Our study provides experimental evidence that microplastics may facilitate protozoan pathogen contamination in shellfish. These results demonstrate how anthropogenic pollution may have unintended consequences on infectious disease transmission in coastal ecosystems, with potential risk to wildlife populations and human public health.