Abstract <p>For the first time, microplastic contamination of the Loire River, from its source to the estuary, was investigated. As part of the Loire Sentinelle project, water and sediment were sampled over a period of three months at 20 stations surrounding major cities and tributaries along this river, which stretches over 1,000&#xa0;km. Microfibers were analysed from water and sediment samples by microscopy and particles from sediment samples by micro-Fourier transform infrared spectroscopy (especially fragments) to characterise their shape, nature, size, and colour. These data were compared to the sediment composition and other abiotic factors. The most striking result is the omnipresence of microplastics all along the Loire River. Microplastic particles found were mainly composed of polypropylene and polyethylene, coloured in blue and black, and present at concentrations of the same order as those found in other European rivers, between 86 and 1,761 mean microfibers/kg dw of sediment and around 0.13 microfibers/m³ of water. The microplastic contamination did not appear to be influenced by sediment grain size but increased at stations downstream of cities compared to those upstream. Both analytical techniques used in this study generated distinct but complementary results enabling a comprehensive assessment of microplastic contamination.</p> Highlights <p>• Water and sediment samples were collected from source to estuary in the Loire River</p> <p>• Microscopy and µ-Fourier transform spectroscopy produced complementary results</p> <p>• Microplastics were assessed as ubiquitous in the river</p> <p>• Contamination increased in stations downstream cities, not dams or tributaries</p> <p>• Sediment composition was not shown to have a high influence on contamination levels</p> Graphical Abstract <p></p>