The Seine estuary is a highly anthropized environment, for which it is necessary to use in situ approaches, in order to measure the level of contamination in estuarine organisms. Many reports have identified microplastics (MP) in aquatic environments and studied their accumulation and effects in various organisms. However, there is a lack of information regarding their presence in estuarine species. The aim of our study was to evaluate the presence of plastic particles in the ragworm Hediste diversicolor sampled from the mudflats of the Seine estuary (France) during April and June 2017 and 2018, on two locations characterized by high anthropogenic pressures (S1 and S2) and one of them with an important hydrodynamism (S2). For each sampling campaign, particles were analyzed after digestion of pools of two worms and two gut contents (depurated sediment) after 24h of depuration. The number of particles as well as their size, shape and color were reported and compared between sampling period and locations. Results showed the presence of particles in both worms and gut content. More particles were observed in gut content with 60.7% in pools from site S1 and 88% for S2. In worms, 40% (S1) and 66% (S2) of the pooled samples contained particles. However, a low number of particles per samples was observed. For example, in worms collected in April and June 2017, 0.75 ± 0.7 and 0.21 ± 0.31 particles/worms were counted. The majority of particles observed were fibers and fragments but foam, films and granules were also identified. In addition, the most polymer type observed by Raman spectroscopy was polypropylene. This study revealed that microplastics are ingested by the ragworm H. diversicolor but more research is needed to measure their effect, especially after chronical exposure in estuarine organisms such as the ragworm H. diversicolor.