Mercury (Hg) is a ubiquitous pollutant that is highly toxic to wildlife, adversely affecting immune, nervous system, behavior and reproduction. Here we provide baseline levels of total Hg in breast feathers of Swinhoe’s Storm Petrels (Oceanodroma monorhis) that breed in Chilbaldo islet. Total Hg concentrations in breast feathers (n = 62) ranged from 0.23 to 8.33 μg/g fw (mean: 1.86 μg/g fw), and five individuals exceeded 5.0 μg/g fw, the threshold level that has been hypothesized to result in adverse reproductive success in seabirds. Seabirds could serve as bioindicators of Hg exposure as they are subject to Hg biomagnification in aquatic ecosystems. Our data emphasize the need for further study of Hg contamination and its potential threats on the globally near-threatened Swinhoe’s Storm Petrels breeding on the southwest coast of Korea.