The Mediterranean Sea (MS) is a large oligotrophic sea whose productivity is sensitive to riverine nutrient inputs. More specifically, phosphorus (P) river supply is crucial for the MS, with an important role of the estuarine/deltaic filter especially for the storage and recycling in sediments. A benthic dataset from the Rhône River prodelta was used to derive P budgets, by means of an early diagenetic model including the benthic P cycle. The model was fitted to pore water profiles of oxygen, nitrate, sulfate, dissolved inorganic carbon, ammonium, oxygen demand units, dissolved inorganic phosphorus (DIP) and solid data (organic carbon (OC), Fe-bound P, Ca-bound P and organic P). Results indicated that the intensity of biogeochemical processes occurring below the sediment-water interface decreased from the river mouth to the adjacent continental shelf with decreasing integrated rates of OC mineralization (160-10 mmol m−2 day−1). The organic P mineralization was intense near the river mouth and decreased offshore (1196-80 μmol m−2 day−1). Its contribution to DIP release was large (> 90%). Fe-bound P had a key role in transferring P to deeper layers. These deltaic sediments played an important role as a source of regenerated DIP. A significant part of DIP was recycled to the overlying waters (72-94%), representing 25% of the riverine DIP discharge. Simultaneously, 6-28% of DIP produced in sediments was buried as Ca-bound P. Overall, this study highlighted the importance of deltaic sediments as an additional source of DIP to the coastal sea, and a minor but permanent sink of phosphorus as solid P burial.