The Antarctic Circumpolar Current (ACC) is the world's largest ocean current system, influencing global ocean circulation, climate, and Antarctic ice-sheet stability. This study documents changes in ACC strength from sediment cores in the Pacific Southern Ocean over the past 5.3 million years. Unlike global cooling and increasing ice volume, the ACC showed no linear long-term trend. Instead, a reversal occurred on a million-year timescale, with ACC strength increasing during Pliocene global cooling and decreasing with further Early Pleistocene cooling. This shift coincided with a Southern Ocean reconfiguration that altered the ACC's sensitivity to atmospheric and oceanic forcings. ACC strength changes were linked to 400,000-year eccentricity cycles, likely modulated by precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability. The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, suggesting potential future increases in ACC flow with global warming. The study highlights the importance of understanding ACC variability for assessing the impact of climate change on the Southern Ocean and global carbon cycle.The Antarctic Circumpolar Current (ACC) is the world's largest ocean current system, influencing global ocean circulation, climate, and Antarctic ice-sheet stability. This study documents changes in ACC strength from sediment cores in the Pacific Southern Ocean over the past 5.3 million years. Unlike global cooling and increasing ice volume, the ACC showed no linear long-term trend. Instead, a reversal occurred on a million-year timescale, with ACC strength increasing during Pliocene global cooling and decreasing with further Early Pleistocene cooling. This shift coincided with a Southern Ocean reconfiguration that altered the ACC's sensitivity to atmospheric and oceanic forcings. ACC strength changes were linked to 400,000-year eccentricity cycles, likely modulated by precessional changes in the South Pacific jet stream linked to tropical Pacific temperature variability. The strongest ACC flow occurred during warmer-than-present intervals of the Plio-Pleistocene, suggesting potential future increases in ACC flow with global warming. The study highlights the importance of understanding ACC variability for assessing the impact of climate change on the Southern Ocean and global carbon cycle.