Microplastic (MP) contamination in marine ecosystems is a rising threat that seriously affects ecosystem health and biodiversity. From sources like industrial wastes, personal care products, and the degradations of larger plastics, MP enters to the oceans via wastewater, runoff, mismanaged waste dumping, and atmospheric deposition. Once such tiny particles enter the marine ecosystem, they could get ingested by marine organisms, which might cause physical blockages chemical toxicity, bioaccumulation, and biomagnification in food chains. MP could alter the habitats chemically and physically within the water column and sediments and alter how an ecosystem functions and the species composition in habitats. This is crucial in coral reef and seagrass bed ecosystems, estuaries and salt marshes, which are most important and sensitive concerning MP pollution. Therefore, habitat degradation, disruption of marine food webs, negative effects on biodiversity, and potential human health risks via bioaccumulation and biomagnification were identified as the most prominent issues that could be led by MP contamination. This requires urgent attention and actions to mitigate MP pollution in marine ecosystems. The chapter highlights the sources and pathways of MP in marine environments, their impact on marine organisms and the ecosystem-level impact, and highlights global hotspots of MP pollution in marine ecosystems.