One of the major phases of sea level rise during the last deglaciation (~19‐11 thousand years before present (ka BP)) is Meltwater Pulse‐1A (MWP‐1A, ~14.5 ka BP), when sea levels rose by 8.6 to 18 meters in less than 400 years. Whether the meltwater originated from the partial disintegration of northern hemispheric (NH) ice sheets, from Antarctica, or both, remains controversial. Here we perform a series of idealized transient simulations of the last deglaciation, focusing on MWP‐1A, with a three‐dimensional oxygen‐isotope enabled Earth System Climate Model. Three meltwater scenarios are considered during MWP‐1A: a sole northern hemispheric source discharging into the North Atlantic, a sole Antarctic source and a combined NH‐Antarctic source. A comparison of simulated changes in the oxygen‐isotope composition (δ18O) of seawater and calcite with published marine sediment records points to a significant contribution from Antarctica. The best model‐data fit is obtained with a contribution from both hemispheres. While the simulated changes over the 350 years of MWP‐1A are overestimated in our simulations, the millennial‐scale changes (~14.6‐13 ka BP) are underestimated, potentially alluding to a longer and sustained meltwater input over the whole period. Meltwater was not applied in the Arctic, the Gulf of Mexico or the North Pacific in our simulations and therefore scenarios with meltwater originating from these regions cannot be excluded.