Sea-surface temperatures (SSTs) reconstructed from Ocean Drilling Program Sites 806 and 849 in the western and eastern equatorial Pacific provide a history of the strength of the Walker circulation between 1.5 and 0.5 Ma. A progressive cooling in the eastern equatorial Pacific is paralleled by intensification of the equatorial zonal SST gradient that began 1.17 Ma, prior to the expansion of global ice volume associated with the mid-Pleistocene climate transition. The SSTs in the eastern equatorial Pacific and the zonal gradient reached modern values by 0.9 Ma. We propose that this onset and intensification of the modern Walker circulation reduced heat flux, but increased moisture transport to high latitudes, leading to the development of more extensive ice sheets and the shift toward the 100 ka world.