Radiocarbon (14C) dating is a widely used and powerful tool for determining the ages of samples in studies of palaeoclimatology, palaeoseismology and archaeology. 14C ages of marine samples often require correction for local reservoir age (ΔR). Although ΔR in the mid‐ to low latitudes has varied through time, few spatiotemporal reconstructions of ΔR have been published for those regions. Here we report on new and previously published ΔR data for the early 1900s to 1950 obtained from coral skeletons from the Kuroshio region off southern Japan. The data presented here show a marked positive‐to‐negative shift of ΔR from the early 1900s to the 1940s, which has important implications for calibration of radiocarbon ages in the region and for understanding the relationship of ΔR in the western Pacific with both the El Niño–Southern Oscillation and the Pacific Decadal Oscillation.