This work shows that the plastic debris accumulated along with stranded Sargassum biomass in Guadeloupe’s beaches contains different forms of arsenic. Results from synchrotron nano X-ray Fluorescence (nanoXRF) and nano X-ray Absorption Near Edge Structure (nanoXANES) show that arsenate (As(V) in a tetrahedral coordination) present in seawater is complexed in the algae cell walls in an octahedral As(V) form, which is subsequently reduced to As(III) within the algae. Inorganic As(III) is either excreted or may undergo methylation and/or binding to glutathione, which is then stored in the algal cells or excreted. The areas where As is colocalized with a variety of metals (Si, K, Ca, Fe, Ni Cu and Zn) may correspond with areas in which algae tissues remain adhered to the surface of the plastics. On the opposite, the areas in which As is found together with Ti or Cl may correspond with areas in which the algae has been decomposed or in which As has been adsorbed after being secreted by the algae. Results from this study should be taken into account to assess the ecotoxicological impacts of Sargassum biomass accumulated on beaches, as well as for the planning of its valorization. Plastics within the Sargassum biomass can act as vectors for arsenic, facilitating its transfer to other environmental compartments where the biomass is used or when it is ingested by various organisms. In a context of a growing problem of plastic pollution and a more and more frequent algae blooms, these results are particularly relevant.