Abstract The role of phytoplankton, particularly diazotrophs, in affecting in situ 210Po scavenging has been little investigated. Here, we first report an enhanced 210Po deficit in a cyclonic eddy core relative to its edge in the subtropical Pacific, coinciding with an elevated integrated nitrogen fixation rate (NFR). In contrast, particulate organic carbon, chlorophyll a, total particulate matter, and 210Pb show no core‐edge difference, revealing a selective fractionation between 210Po and 210Pb during scavenging. The partition coefficient of 210Po, but not 210Pb, correlates with NFR. The 210Po/210Pb fractionation factor also scales with NFR. These relationships indicate diazotrophs’ uptake of 210Po and provide a field data‐based algorithm linking 210Po disequilibria to NFR in (sub)tropical oceans. Our findings suggest that 210Po offers a potential radiotracer for quantifying marine NFR, highlighting a new insight to constrain nitrogen cycling in oligotrophic oceans.