Massive Sargassum sp. beachings have been occurring on Caribbean shores since 2011. The sargassum involved in such events are S. fluitans and S. natans, two drifting species whose proliferation has been observed in the southern North Atlantic Ocean. Both for reasons of environmental and sanitary assessment and repurposing, Sargassum sp. that is ashore piled up on beaches and decaying must be studied. Studies are required because of the concerning content of pelagic arsenic reported in the literature. They are also needed owing to Sargassum sp. contamination subsequent to historical pollution in the French West Indies by chlordecone, an insecticide used against the banana weevil Cosmopolites sordidus. The present study aims to describe the contamination and decontamination toxicokinetics of arsenic and chlordecone for Sargassum sp. stranding on shores and shallows in the Caribbean, in order to support the decision-making of the authorities involved. In situ and in mesocosm experiments performed in the present study show that Sargassum sp. contamination by chlordecone is mainly done after 2 h of exposition and reaches equilibrium after a day of exposure in polluted water, but BCF study suggests that the phenomenon is not actively supported (passive soption only). Arsenic transudation is intense in the case of immerged algae both. Half of the arsenic content is transudated after 13 h at sea and will transudate until vestigial arsenic concentration. Sargassum sp. contamination by arsenic, due to phytoaccumulation offshore, is broadly homogeneous before decay, and then leaks lead rapidly to a decrease in concentration in Sargassum sp. necromass, questioning the subsequent contamination of the coastal environment.