During cruise MSM41 of the German research vessel Maria S. Merian in April 2015 a visual ship based survey was conducted on the composition and density of natural and anthropogenic flotsam in the Sargasso Sea (subtropical NW Atlantic). On transits between the regular stations of the cruise, flotsam was quantified on transects of 10 m width and lengths of 9.1 - 25.5 km. Floating marine debris items passing through the transect was recorded and specified (item type, size and color) when possible. The position of each floating item was determined as the position of the observer at the moment the item was registered using a handheld GPS. Clumps of pelagic Sargassum were too numerous to be counted individually throughout an entire transect. Instead, numbers of clumps passing through the transect area were counted within time intervals of 30 seconds, interrupted by breaks of 30 seconds. In extremely dense accumulations or windrows of pelagic Sargassum, the number of clumps was estimated in broader but less accurate entities (i.e., tens or hundreds of clumps). The estimated minimum size of items identifiable from the moving ship was 2 cm. Each flotsam item was assigned to a material category (e.g., plastics, Styrofoam, metal).