Understanding the coupling between sediment availability and sea-level change is important for forecasting coastal-barrier (barrier islands and barrier spits) response to future sea-level rise (SLR). An extensive data set of sediment cores, seismic profiles, and a high-resolution chronology from the Wadden Sea (southeastern North Sea) documents that long-term barrier-chain progradation was interrupted by a period of widespread barrier deterioration between ca. 3.5 and 2.0 ka. The decay of the barrier islands resulted from a decrease in littoral drift triggered by regional-scale coastal reconfiguration. The formation of a large cuspate foreland updrift caused the depositional locus to shift away from the barrier coast. Our results demonstrate that the resulting reduction in marine sediment availability substantially decreased the stability of the barrier chain, causing the regional SLR thresholds to fall from between 2 and 9 mm yr–1 to ~0.9 mm yr–1, and thus below contemporary rates of SLR. Hence, we argue that predicting the response of barrier coasts to ongoing SLR requires consideration of possible changes in sediment availability and the role of large-scale geomorphological feedbacks due to human and natural causes.