Here, we present the first combined results of N₂ fixation rates (15N₂ assay), dissolved iron (dFe, < 0.2 μm), and primary production (PP) (14C assay) in the northwestern South China Sea (NWSCS) coastal upwelling region during summer. Surface N₂ fixation rate ranged between 0.1 nmol N L−1 d−1 and 5.6 nmol N L−1 d−1 (average 1.0 nmol N L−1 d−1, n = 50) under nonbloom conditions. At a Trichodesmium bloom station, N₂ fixation rate was ∼ 3 orders of magnitude higher. Depth-integrated N₂ fixation rate ranged between 7.5 μmol N m−2 d−1 and 163.1 μmol N m−2 d−1 (average 46.4 μmol N m−2 d−1). Our results indicate that N₂ fixation is unlikely limited by Fe availability in the NWSCS continental waters, instead, the coastal upwelling-induced combined effects of physical and biological processes may have played a decisive role. With the upwelled cold, dFe-rich, nutrient-replete waters, nondiazotrophic phytoplankton growth would be preferentially enhanced while N₂ fixation was hindered due to relative deficiency of phosphate caused by massive phytoplankton utilization in the coastal upwelling. By comparison, N₂ fixation was notably elevated along with decreased PP in the offshore waters, probably due to a shift from P-deficiency to N-deficiency. Consistently, the contribution of N₂ fixation to PP (0.01–2.52%) also increased toward the open waters. As a significant external N source, summertime N₂ fixation is estimated to contribute a flux of 1.4 Gmol N to this area under nonbloom conditions. This study adds to the knowledge of N₂ fixation in the rarely studied subtropical coastal upwellings, and highlights the necessity of future comprehensive studies in such highly dynamic environments.