Pelagic Sargassum and floating marine debris were collected during cruise MSM41 of the German research vessel Maria S. Merian in April 2015 in the Sargasso Sea, western subtropical North Atlantic. In total 39 debris samples and 42 Sargassum samples were taken and characterized. The associated biota was analyzed. Mobile species were counted, sessile organisms were recorded as present/absent. A visual ship-based survey was conducted to quantify pelagic Sargassum and floating marine debris in the Sargasso Sea. Biomass and surface area of Sargassum was measured from thalli beached at Sisal Beach, Yucatan peninsula, Mexico to establish the biomass:surface area ratio for the two species Sargassum fluitans and Sargassum natans.