Does marine litter mainly come from the land? Does population density influence the discharge of land-based litter? To answer these questions, we conducted a bicycle survey in 2020 to assess the hotspots of litter along the 242.5 km of the Tamsui River system and 39 km of its adjacent coastline. The Tamsui River is the largest river system in northern Taiwan and ranked 16th in the Ocean Cleanup’s research as one of the most polluting rivers. Our survey, conducted with 24 volunteers and the NGO “Society of Wilderness”, revealed notable differences between the litter at the river mouth (coastline) and the riverbanks. Litter density was much higher at the river mouth (15.3 m3/km) than the riverbanks (0.2 m3/km to 2.8 m3/km). The coastline was mainly polluted by derelict fishing gears, whereas single-use plastics and illegal dumping household waste occupied the upstream areas, suggesting that coastal litter is more likely coming from ocean-based activity rather than land-based sources. Furthermore, our results showed no correlation between litter and population density. The survey was published in Sustainability.