Ocean acidification (OA) caused by increasing atmospheric CO₂ concentration will lead to a variety of effects on marine phytoplankton and primary production. However, the response of major primary producers (i.e., picophytoplankton) and primary production to OA in oligotrophic tropical and subtropical areas remains poorly understood. In this study, we conducted 47 onboard microcosm experiments (10–20 L; 2–3 day) at 43 stations throughout the (sub)tropical western North Pacific, spanning from the Northern South China Sea (NSCS) to the North Pacific Subtropical Gyre (NPSG). We observed distinct regional and seasonal variability in responses of primary production to OA across the study area. In the NPSG, rates of primary production in warm seasons decreased by 31.2 ± 13.0 % under acidified conditions. In comparison, primary production in the NPSG during winter and in NSCS basin were less sensitive to OA (on average decreased by 15.1 ± 7.0 % and 15.5 ± 9.6 %, respectively). These differences were related to shifts in phytoplankton community compositions and nutrient availability. In the study area, the dominated by picocyanobacteria, Prochlorococcus and Synechococcus, were the most abundant primary producers but played a minor role in the OA-induced decrease in community primary production. By contrast, the OA impacts on the abundance of picoeukaryotic phytoplankton in each of the study region were highly consistent with those on the overall primary production. In addition, we found a strong positive correlation between OA-induced relative changes of primary production and averaged ambient chlorophyll-a concentrations, indicating that the effect of OA on primary production may be regulated by nutrient availability. Overall, our study revealed the impact of OA on primary production in the tropical western North Pacific, and the potential roles of different phytoplankton populations and nutrient availability played in it.