The Thames has a large catchment, 16,000 km2 encompassing 15 million residents and passes through major urban centres e.g., Oxford, Reading and London. Consequently, the Thames is vulnerable to domestic and industrial wastewater pollution. It is one of the most microplastic-contaminated rivers in the UK and this high contamination is likely to have a detrimental effect on wildlife in the river. This study presents evidence for microplastic contamination in two resident species of brachyuran crab from the Thames Estuary: the native shore crab, Carcinus maenas (Linneaus, 1758) and the invasive Chinese mitten crab, Eriocheir sinensis (H. Milne Edwards, 1853). Crabs were sampled periodically (ca. every three months) between December 2018 and October 2019 and their gills, gastric mill and gastrointestinal tract examined. A total of 94 C. maenas and 41 E. sinensis were collected and 874 pieces of plastic recovered, ranging 34 μm–34 mm in length. Overall, 71.3% and 100% of C. maenas and E. sinensis, respectively, contained at least one item (fibre, film, fragment or tangle of fibres) in the gill chamber, gastric mill or gastrointestinal tract. The most common items were fibres (78.5%) but in some cases, particularly in the gastric mill, these were aggregated into tangles (7.8%). Almost all E. sinensis contained tangles of fibres (95.1%), whereas, relatively few C. maenas contained similar tangles (10.6%). This work was supported by the National Environment Research Council [grant number NE/ L002485/1] with co-sponsorship from a Fishmongers' Company's Fisheries Charitable Trust CASE Partnership.