Measurements of the partial pressure of CO₂ in surface seawater (pCO₂^w) have been made frequently and extensively in the western North Pacific (3-35°N, 132-142°E) since 1990. Based on the time series analysis of pCO₂^w data, we obtained a “climatological view” of seasonal variation in pCO₂^w in the western North Pacific. We have examined the relationship between pCO₂^w and sea surface temperature (SST). The pCO₂^w–SST relationship varies spatially and temporally. The pCO₂^w showed an average growth rate of 1.6 µatm yr^-1 (nearly equal to that of the air, pCO₂^a) with large variability (±8.9µatm yr^-1). In 1998, larger growth rates of pCO₂^w occurred in the subtropical gyre and the western equatorial Pacific, which was probably associated with the 1997/98 El Niño phenomena. To know processes affecting long-term variations in pCO₂^w, we have examined seasonal variation in growth rate of pCO₂^w. The linear growth rate of pCO₂^w during the winter season ranged from 1.3±0.2 to 2.1±0.2µatm yr^-1 with an average of 1.7±0.2µatm yr^-1. During spring/summer seasons, the average growth rate of pCO₂^w was larger than 2µatm yr^-1 north of 27°N, and within the range from 0 to 1µatm yr^-1 in the North Equatorial Current. These increases were mostly caused by the oceanic uptake of anthropogenic CO₂, and to some extent, other processes controlling the pCO₂^w change: thermodynamic effect, lateral transport and vertical mixing, and biological activity.