This dataset provides numbers and types of plastic particles extracted from sediment samples of three tributaries of the river Thames: the River Leach, the River Lambourn and The Cut. These rivers are regularly monitored for a range of water quality and biological characteristics as part of the ongoing CEH Thames initiative project. Four sampling sites were selected based on the average percentage of effluent present in the river and population equivalent density to represent scenarios ranging from low sewage input and population equivalent density (Leach and Lanbourn) through an intermediate site (the Cut) to a site with high sewage input and population equivalent density (also in the Cut). The samples were collected between late August and early September 2014. The data provides information on the site characteristics, dry weight of sediment analysed (in grams), number of microplastic particles extracted and characteristics of particles (including shape, colour and polymer type). Types of polymers identified include: polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyethylene terephthalate, nylon and polyester. Full details about this dataset can be found at https://doi.org/10.5285/93837492-408f-4349-8dcd-ee833e84e47e