<p>The sources and seasonal trends of water-soluble organic carbon (WSOC) in carbonaceous aerosols are of significant interest. From July 2017 to July 2019, we collected samples of PM<sub>2.5</sub> (particulate matter, aerodynamic diameter < 2.5 μm) from one suburban and one rural site in Japan. The average δ<sup>13</sup>C<sub>WSOC</sub> was −25.2 ± 1.1 ‰ and −24.6 ± 2.4 ‰ at the suburban site and rural site, respectively. At the suburban site, the δ<sup>13</sup>C<sub>WSOC</sub> was consistent with the δ<sup>13</sup>C of levoglucosan, a tracer of biomass burning, and a high correlation was found between WSOC concentration and non-sea-salt potassium concentration, another tracer of biomass burning, suggesting that the main source of WSOC was biomass from burning of rice straw. At the rural site, the δ<sup>13</sup>C<sub>WSOC</sub> was significantly heavier in winter (−22.6 ± 1.3 ‰) than in summer (−27.4 ± 0.7 ‰; <i>p</i> < 0.01). The heavy δ<sup>13</sup>C<sub>WSOC</sub> in winter was a result mainly of biomass burning and the aging of OC during long-term transport, whereas the light δ<sup>13</sup>C<sub>WSOC</sub> in summer was a result mainly of the formation of secondary organic aerosols from biogenic volatile organic compounds. Thus, our δ<sup>13</sup>C<sub>WSOC</sub> approach was useful to elucidate the sources and atmospheric processes that contribute to seasonal variations of WSOC concentrations.</p>