The Antarctic Circumpolar Current (ACC) is a key component of Earth¿s climate system due to its strong influence on the global overturning circulation, ocean heat and CO2 uptake and storage. The timing and mechanisms of the development of a homogeneous and deep circumpolar flow remain disputed, despite the critical significance of the ACC for Earth¿s climate. Here we present neodymium isotope and sortable silt records from two pelagic sedimentary archives in the Southwest Pacific and South Indian Ocean (DSDP 278 and ODP 744, respectively) spanning the last 31 million years (Ma). Our data demonstrate that a modern-like deep-reaching circumpolar current did not exist before the Middle Miocene. This contrasts the long-held convictions that the development of a strong, deep ACC was solely controlled by the opening and deepening of Southern Ocean gateways, and resulted in significant glaciation across the Eocene-Oligocene Transition (34 Ma). We suggest that besides tectonic pre-conditioning, the expansion of the Antarctic ice sheet and sea ice during the Middle Miocene Climate Transition played a vital role in driving intensification of Southern Westerly Winds and establishment of a deep-reaching circumpolar flow.