Abstract The nature and cause(s) of the Mid‐Pleistocene Transition (MPT) remain enigmatic. Notably, the role of tropical Pacific oceanographic changes during the late MPT (∼0.9–0.6 Ma) remains understudied. Here, we generated ∼1.4‐Myr‐long Mg/Ca and δ18O records for the planktonic foraminifer Trilobatus sacculifer from IODP Site U1490 to reconstruct sea‐surface temperature and regional seawater δ18O (ice‐volume‐corrected, a proxy for salinity) variation in the Western Pacific Warm Pool (WPWP). We find that the WPWP was characterized by a gradual warming and slight freshening of surface waters at ∼0.9–0.6 Ma, caused by the Indonesian Throughflow weakening and consequent accumulation of oceanic heat. These effects were likely driven by sea‐level fall due to global icesheet expansion and/or Australian monsoonal intensification. The gradual warming signal was subsequently propagated eastward via strengthened North Equatorial Countercurrent, which may have acted as a positive climatic feedback, contributing to the expansion of the North American icesheet during the late MPT.