The purpose of this paper is to study denitrification and the conditions for its development in a hyporheic zone. The study site is the riparian zone of a former branch of the Seine River, where the river stage is kept almost constant during the year by hydraulic regulation. Hydrological and geochemical surveys were performed by monitoring four wells, ten shorter piezometers and the river over a 15-month period. The water fluxes originating from the chalky hillsides and the river converge in a zone parallel to the river that acts as a drainage flow path through the floodplain. The riparian zone between this flow path and the river shows an important depletion of nitrate during the summer and autumn period, which cannot be explained by a simple mixing of waters coming from the river and the chalky hillsides. It can be attributed to denitrification as it occurs when oxygen concentration is below 2 mg l1, and goes along with a consumption of dissolved organic carbon and a decrease of redox potential. The river completely controls these hydro-geochemical conditions. It also keeps the wetness of the riparian zone almost constant, which allowed us to isolate the high temperatures in summer and autumn as an important triggering factor for denitrification through its influence on the reaction rate and oxygen deficits. We also found a small isotopic enrichment of nitrate, suggesting that denitrification occurs after diffusion of nitrate through the sediment and riparian zone matrix, which is consistent with the hyporheic functioning of the study site.