The south-eastern Bay of Biscay is an accumulation zone for marine litter. Yet, the behaviour of the riverine litter fraction reaching the sea is poorly understood. We resolve this by studying litter buoyancy and transport, based on high-frequency radar observations and Lagrangian simulations. We show large seasonal and regional differences between items' behaviour, particularly in summer when highly buoyant litter beaches faster and Gipuzkoa and Pyrénées-Atlantiques regions are mostly affected.