Sediment cores were collected at the outlet of the highly anthropogenized catchment of the Seine River at two contrasting sites: a flood plain of the lower Seine River and a quasi-permanently submerged harbour basin (or wet dock) in the upper tidal estuary. Analyses of artificial radionuclides (137Cs and plutonium isotopes), coupled with hydrological and bathymetric data, lead to a precise dating of the sediment cores collected at the two sites. 137Cs signals originating from global fallout (early 1960s) and from the Chernobyl accident (1986) are identified, but at different levels due to the incomplete nature or variable continuity of the records. Anomalous 238Pu concentrations found at both sites (1–2 Bq kg−1) are attributed to unknown industrial releases originating from upstream. Interpolating 137Cs sediment activities under the assumption of a constant sediment rate, those releases were dated back to 1975 ±1, thus providing a local but reliable time-marker. Age models have highlighted a very contrasting sediment filling dynamics in these two sites. This study presents the first sediment record of alpha- and gamma-emitting artificial radionuclides obtained at the outlet of the huge catchment area of the River Seine, over a period covering the last 50 years. •The sediment dynamics is studied at two contrasting sites in the Seine River.•137Cs, 238Pu an 239,240Pu profiles are provided and used as time markers.•Radionuclide and hydrodynamics data are combined to build up accurate age models.•It is the first step for a reconstruction of the Seine River contamination history.