AbstractThis study examines the oxygenation efficacy of geoengineered artificial downwelling (AD) in the hypoxia zone of the Pearl River Estuary (PRE) using a fine‐resolution coupled physical‐biogeochemical model. Like many estuaries worldwide, the PRE experiences summer hypoxia because of intense stratification and eutrophication. AD provides a potential solution for short‐term hypoxia relief via the mixture of surface and bottom water, but its oxygenation efficacy relying on both local hydrodynamics and biogeochemistry is largely uncertain. Our simulations suggest that AD has extremely low oxygenation efficacy in the PRE during the hypoxia period. The estimated oxygen transfer efficiency (OTE), defined as the rate of oxygen transfer per unit of power needed for pumping, varies between −0.81 and +0.92 kg O2 kWh−1. This is an order of magnitude less than the previous estimate of 10–100 kg kWh−1 in closed lakes and fjords. Moreover, a negative OTE means that AD sometimes backfires and exacerbates hypoxia in the bottom water. The above results remain unchanged when different values of engineering parameters including downwelling rate and discharging depth are used. Extremely low oxygenation efficacy is attributed to strong subtidal advection in the estuary as well as the engineering side effects including reduced vertical diffusion.