Ultra-processed foods now dominate the food supplies of high-income countries, with over 50% of energy intake coming from ultra-processed foods in the United States. Observational data has revealed that greater ultra-processed food consumption is associated with adverse mental health outcomes, while data from randomized controlled trials has demonstrated improvements to mental health following reduction in ultra-processed food intake. Ultra-processed foods are known to contain high concentrations of microplastics, largely due to both the processing and packing procedures. In light of recent findings which demonstrated alarming microplastic concentrations in the human brain, we propose that microplastics may partially mediate the adverse mental health effects of increasing ultra-processed food intake. In this viewpoint, we discuss the overlapping mechanisms for adverse mental health, paucity of research in the area, and propose a Dietary Microplastic Index (DMI) to study this potential relationship.