Marine microplastics have become pervasive contaminants in oceans, sediments, and marine organisms due to the rapid growth of global plastic production and widespread mismanagement of plastic waste. Their small size, diverse polymer composition, and ability to adsorb pollutants enable them to enter marine food webs and reach humans primarily through the consumption of fish, bivalves, crustaceans, and other seafood. Microplastics impair marine organisms by inducing physiological stress, inflammation, reduced growth, altered feeding, and reproductive disruptions, ultimately decreasing seafood availability and quality. Human exposure through diet is accompanied by toxicological risks, including gastrointestinal disruption, oxidative stress, microbiome imbalance, endocrine interference, and the bioaccessibility of sorbed contaminants. These effects undermine all four pillars of food and nutritional security, availability, access, utilization, and stability, particularly in seafood-dependent communities. Climate change further accelerates plastic fragmentation, increases chemical leaching, redistributes microplastics through extreme weather events, and amplifies combined toxicity in marine ecosystems. Effective monitoring, harmonized standards, global governance initiatives such as the UNEA-mandated plastics treaty, and circular-economy interventions are essential to mitigate current and future risks. This chapter synthesizes environmental, toxicological, and socio-economic dimensions to outline how microplastics challenge human nutritional security.