Marine litter is currently one of the most widespread pollutants. However, monitoring the spatial-temporal changes of floating marine litter remains challenging. In this study, with data collected by citizen scientists, we investigated the distribution and density of floating marine litter in the waters around Taiwan. With 40 volunteers surveying 978 transects in 410 cruises from December 2017 to December 2018, the overall survey area was approximately 121.26 km(2) (5455 km in length). We found that the mean density of floating marine litter was 90.10 +/- 8.60 items/km(2) and that the majority was plastic (63.2%). Nearly 70% of the litter was found in 30% of the surveyed area. We also identified hotspots of floating marine litter, which are at the Northwestern and Southeastern regions of Taiwan. Our study highlights the contribution of citizen science to monitor floating marine litter and provides insight into the management of marine litter.