The ingestion of microplastics has been recorded in hundreds of species. The ingestion rate and degree of impact is species-specific and depends on food preferences, foraging behaviour, and plastic pollution of the area. Currently there is a large knowledge gap regarding ingestion of marine litter by invertebrates in brackish water bodies. Therefore, experiments were conducted to investigate microplastics uptake and potential accumulation in the digestive system of the Harris mud crab Rhithmpanopeus harrisii. Effects of microplastics on the growth of crabs were also tested. The results show that R. harrisii consume microplastics together with food, only plastic fragments too large for the digestion system were removed by crabs. The effect and duration of passage of plastic are not consistent and depend on the size and type of plastic. Microplastic fragments (<0.25 mm) ingested by crab were continuously egested while knotted fibres may accumulate in the stomach. The crabs fed with plastic supplements lost weight or had lower weight increase compared to control group after two-month treatment. However, the differences between treatments were not statistically significant. For the first time, occurrence of plastic in the digestive system of crabs collected from the natural habitats of Parnu Bay (NE Baltic Sea, Estonia) was studied. Among all the crabs examined, 5% of specimens were found with fibres in their stomach assimilated prior to their capture from nature.