Plastic pollution is a major issue affecting the oceans. Despite rivers being the principal source of plastic debris, few of the studies on plastic pollution are focused on freshwater environments. To contribute to filling this knowledge gap, we analyzed three years of data on plastic pollution in the Thames in London. Using a community ecology approach, we analyzed abundance and composition of plastic items, testing for changes across time and space. We found two different types of sites with different compositions of items and a stable abundance of macro-plastic debris in the years 2016–2018. We identified a list of statistically significant plastic items as potential representative indicators in future surveys to estimate what is present in the water column as well as conduct effective cleaning and protection efforts. This study aims to optimize future data collection efforts, creating a reference for forthcoming similar work around the globe especially in high-income, high-density urban areas, with implications for policymakers, non-governmental organizations and researchers.