Seawater U-234/U-238 provides global-scale information about continental weathering and is vital for marine uranium-series geochronology. Existing evidence supports an increase in U-234/U-238 since the last glacial period, but the timing and amplitude of its variability has been poorly constrained. Here we report two seawater U-234/U-238 records based on well-preserved deep-sea corals from the low-latitude Atlantic and Pacific Oceans. The Atlantic U-234/U-238 started to increase before major sea-level rise and overshot the modern value by 3 per mil during the early deglaciation. Deglacial U-234/U-238 in the Pacific converged with that in the Atlantic after the abrupt resumption of Atlantic meridional overturning. We suggest that ocean mixing and early deglacial release of excess U-234 from enhanced subglacial melting of the Northern Hemisphere ice sheets have driven the observed U-234/U-238 evolution.