Stable isotope ratios of carbon and nitrogen (δ¹³C and δ¹⁵N) in the particulate organic matter (POM) of the euphotic layer were seasonally investigated in the vicinity of the Kuroshio from 2008 to 2019 (n = 490). Generalized linear models (GLMs) showed significant seasonality of δ¹³C and δ¹⁵N of POM in the coastal (between Japan Main Island and the northern edge of the Kuroshio) and offshore (Kuroshio and more southern parts) areas. Seasonal climatological δ¹³C estimated based on the GLMs and seasonal median values of the environmental parameters was the highest in summer (-22.4 ± 0.2‰ and -22.9 ± 0.2‰ in the coastal and offshore areas, respectively) and lowest in winter in both areas (-23.9 ± 0.2‰ and -24.3 ± 0.2‰ in the coastal and offshore areas, respectively). Seasonal climatological δ¹⁵N showed different spatial variations from spring to summer. The δ¹⁵N value was the lowest during winter (0.8 ± 0.4‰), and increased to a similar level during the other three seasons in the coastal area (~3‰), suggesting that nitrate originating in the deep-sea water was the main source of new production from spring to autumn. In contrast, δ15N in the offshore areas decreased from spring (2.6 ± 0.4‰) to summer (0.9 ± 0.4‰), suggesting that the significant contributions of atmospheric deposition and nitrogen fixation in the summer were similar to those around Hawaii. Therefore, the nitrogen sources for biological production were different between the areas and seasons in the vicinity of the Kuroshio.