Marine pollution is the most challenging and inadequately addressed global environmental issue. The rapid increase in industrial and agricultural growth coupled with technological advancements has introduced massive emissions and discharges of hazardous wastes in the biosphere. Over time humans have exploited the marine ecosystem as a major sink of pollution from both land and water sources. Ocean pollution is the mixture of heavy metals, organic pollutants, pharmaceutical residuals, plastics, and oil spills. This chapter investigates the migration and chemical transformation mechanisms of major marine pollutants including heavy metals, organic pollutants, pharmaceutical residuals, plastics, and oil spills. Metals mostly settle and adsorbed on sediments at the bottom which act as secondary source and may resuspend in marine environment through desorption. Chemical transformation of heavy metals is dependent on its chemical forms as metals in exchangeable fraction are highly active whereas residual fractions are observed to be relatively stable in marine environment. Deep-sea ecosystem acts as sinks for organic pollutants and biomagnification leads to build-up of pollutants in marine organisms. Microbial transformation of organic pollutants helps in the removal of organic pollutants in the deep sea through aerobic hydrocarbon degradation. The primary mechanism for the abiotic transformation of medicinal chemicals in the aquatic environment is photochemical degradation. Plastics in the marine environment may undergo both abiotic and biotic degradation processes while plastic additives are also released along with degradation mechanisms and likely to migrate into species through leaching, evaporation, abrasion, and ingestion. Oil droplets when released into marine environment go through complex biological, physical, and chemical transformations which include spreading, drifting, dispersion, stranding, and weathering. The transformation processes of pollutants in marine ecosystem help to further develop insights into the toxic effects produced by pollutants in the marine ecosystem.