Freshwater pulses draining into the North Atlantic Ocean are commonly hypothesized to have perturbed the Atlantic meridional overturning circulation (MOC), triggering abrupt climate changes such as Heinrich events, the Younger Dryas, and the 8.2ka event. However, dating uncertainties have prevented causal links between freshwater pulses and climate events from being firmly established. Here we report a high-resolution relative sea-level record from the Mississippi Delta that documents a sea-level jump that occurred within the 8.18 to 8.31ka (2σ) time window and is attributed to the final drainage of proglacial Lake Agassiz–Ojibway (LAO). This age is indistinguishable from the onset of the 8.2ka climate event, consistent with a nearly immediate ocean–atmosphere response to the freshwater perturbation. This constitutes a rare currently available example of a major abrupt climate cooling that can be directly linked to a well-documented freshwater source with a temporal resolution on the order of a century. The total inferred eustatic sea-level rise associated with the very final stage of LAO drainage at 8.2ka ranges from 0.8 to 2.2m, considerably higher than previous estimates. These new constraints on the timing and amount of final LAO drainage permit significantly improved quantitative analysis of the sensitivity of MOC to freshwater perturbation, a crucial step toward understanding abrupt climate change.