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Since 2011, unusually large quantities of pelagic Sargassum fluitans and S. natans (sargasso) have been washing ashore along the coasts of some African countries and the Greater Caribbean, impacting ecosystems and economies. We estimated biomass and composition of sargasso arriving to a Mexican Caribbean coast from September 2016 until May 2020. In 2016, the beached masses comprised S. natans VIII and S. fluitans III. S. fluitans III was the predominant form throughout the study period, comprising on average >60 % of total wet biomass. The relative abundance of S. natans VIII decreased in time from 2016 to 2019 (41 to 3 %), although it became prevalent again in the first months of 2020. The third morphological form, S. natans I, was not registered until February 2018, and its relative abundance increased from 23 % in 2018 to 31 % in 2019. The initial composition of Sargassum species and morphotypes of the beached sargasso in Mexico differed from that commonly reported in the Sargasso Sea. The total biomass of beached sargasso varied considerably among years and seasons, with peaks during the summer months of 2018 and 2019. Seasonal variations in biomass were explained by 1) presence of sargasso in the Yucatan Current and 2) prevailing trade winds. This study is the first report on biomass and species composition of beached sargasso in the Western Caribbean and may help to understand patterns of the massive influxes; which will aid in the management of this new phenomenon.