Accelerated degradation of aquatic ecosystems caused by excessive nitrogen input has focused more attention on denitrification, which can permanently convert reactive nitrogen (N) into dinitrogen (N₂). While denitrification has been extensively studied, information is limited concerning the specifics of its changing nature on a daily scale. This article presents our development of a simple model to quantify hourly denitrification at the reach scale, which was applied to a flow-through river wetland in Southeast China. By measuring the concentration of dissolved N₂ at the inlet and outlet of the wetland and accounting for the air–water physical exchange, the model can obtain a complete picture of the diel variability during denitrification. Our results showed that denitrification rates varied significantly and exhibited obvious day-and-night differences. The mean denitrification rate during the night reached 12.34 mmol m^-2h^-1, which was three times greater than the daytime rate. Changes in dissolved oxygen and redox conditions were found to be responsible for the observed denitrification dynamics of the wetland. The apparent diel pattern of denitrification as exhibited in the riverine wetland provides a strong rationale for further recognition of denitrification’s instant responses to changes in various environmental factors. Future efforts should also consider taking advantage of the whole-system method as proposed here and conducting more in-situ measurements with wider spatio-temporal coverages to better elucidate time-immediate denitrification processes, and also to facilitate effective management strategies.