We examined primary production (PP) and the assimilation rates of nitrate (ρNO3-), ammonium (ρNH4+) and dinitrogen (ρN2) using 13C and 15N tracer techniques along bay to offshore transects in the Oyashio-Kuroshio-Tsugaru Warm Current interfrontal region in the northwestern North Pacific Ocean, during five research cruises covering a full seasonal cycle. Surface inorganic N was depleted in the bays and the offshore region from summer to fall, but the higher particulate organic carbon (POC)/particulate nitrogen (PN) ratio and higher maximum N assimilation rate observed kinetics experiments in the bays suggest that N limitation was stronger in bays than offshore. PP (295–19,200 nmol C L−1 d−1), ρNO3- (9.45–650 nmol N L−1 d−1) and ρNH4+ (11.6–494 nmol N L−1 d−1) in the surface waters were generally low in summer and high in spring. Different from the cases of ρNO3- and ρNH4+, ρN2 was high (up to 12 nmol N L−1 d−1) in mid-summer, especially in offshore regions, and moderate or low (≤2.26 nmol N L−1 d−1) during other seasons. N2 fixation largely contributed to total new production (up to 29%) in mid-summer. The mean f-ratio, estimated as the ratio of (ρNO3- + ρN2) to (ρNO3- + ρN2 + ρNH4+) at two offshore stations varied within the range of 0.29–0.83 (mean 0.54 ± 0.20). Comparison of the PP and total N assimilation (ρNO3- + ρN2 + ρNH4+) indicated that the ratio of C to N assimilation rates (max. 95) far exceeded the Redfield ratio (6.6). This finding suggests that other sources of N (e.g., dissolved organic nitrogen) contributed significantly to total N assimilation; if this is correct, the estimated f-ratio may be too high. Alternatively, the fC-ratio was estimated from the ratio of (ρNO3- + ρN2) to PP by assuming the Redfieldian assimilation ratio, and it ranged from 0.06 to 1.10 (mean 0.45 ± 0.31). Despite uncertainties in both the f- and fC-ratio estimation, our data provide evidence that a large fraction of PP is potentially available for export to deep waters and to higher trophic levels in the interfrontal region of the western North Pacific.