Polypropylene, polyethylene, polystyrene, polyvinyl chloride, polyamide, and polyethylene terephthalate are the few common polymers used to make plastic. Among these, polystyrene is the most commonly used plastic polymer made by polymerization of styrene monomers. It is widely used in the production of Styrofoam, foodservice packaging, toys, medical devices etc. A growing evidences suggested that polystyrene plastic is an emerging environmental pollutant which is becoming a serious environment problem. Different types of polystyrene plastic pollution, including megaplastic, macroplastic, mesoplastic, microplastic (MPs), and nanoplastic (NPs), are observed across habitats. Numerous marine organisms like bivalves, Caenorhabitis elegans, shrimp, and fishes confounded these MPs and NPs as their food particles easily due to their small size. Additionally, these MPs and NPs have the ability to go up the food chain from lower to higher nutritional levels, endangering human health. Once consumed, these plastics can move to different body organs like kidney, lungs, brain etc. and accumulate there. Some researchers also reported the molecular mechanism of MPs and NPs toxicities like neurotoxicity, hepatotoxicity, reproductive toxicity etc. In fact, due to their small size, NPs can pass through biological membranes, penetrate into tissues, accumulate in organs, and cause injury. This chapter summarized the hazardous risks and associated mechanisms of Polystyrene MPs and NPs on marine life and humans. In addition, possible pharmacological approaches to minimize polystyrene-induced toxicity are also discussed.