Subglacial water that is widely distributed beneath the Antarctic Ice sheet (AIS) plays a key role in ice-sheet instability and catastrophic ice-stream discharge. Proxy data that record the deglacial evolution of subglacial discharge from AIS following the Last Glacial Maximum (26,000-19,000 years ago) are crucial for pinpointing the temporal and causal relationships between subglacial discharge, AIS retreat, and deglacial sea-level rise. Marine records of seawater uranium isotope (²³⁴U/²³⁸U) provide a novel way to document the subglacial discharge due to the accumulation of large amounts of recoiled ²³⁴U in subglacial water. Seawater ²³⁴U/²³⁸U records from the North Atlantic Ocean have been used to show the release of this excess ²³⁴U pool along the collapse of the Laurentide ice sheet, but comparable records from the Southern Ocean of the AIS are still lacking. Here we present absolutely-dated, high-resolution Southern Ocean seawater ²³⁴U/²³⁸U records over the last 30,000 years based on well-preserved deep-sea corals from the Drake Passage. Distinctively higher seawater ²³⁴U/²³⁸U values are found from 15,400 to 14,000 years ago, which matches the timing of the highest iceberg-rafted debris flux from AIS and meltwater pulse 1A (MWP-1A) event, indicating the causal link between enhanced subglacial discharge, AIS retreat, and rapid sea-level rise during MWP-1A. Combined with deep-sea coral Li/Mg-based subsurface temperature record, we postulate that enhanced subglacial discharge and subsequent AIS retreat is likely preconditioned by a stronger and warmer Circumpolar Deep Water, which lends support to an oceanic forcing of AIS retreat during the last deglaciation.