The Younger Dryas stadial (Greenland Stadial 1) has long been recognized in proxy records throughout the North Atlantic region as a stable, cool period that started and ended abruptly. However, modeling experiments in which the Younger Dryas cooling was forced by meltwater predict much more unstable climatic conditions compared to the proxy records. Here we present evidence of a climatically unstable Younger Dryas based on a decadal-resolution marine proxy record from the Norwegian continental shelf. This record shows that the North Atlantic was characterized in the early Younger Dryas by Arctic Water and an increasing accumulation of fresh surface water that peaked at 12,500 calendar yr B.P. For ∼300 yr during the late Younger Dryas, the sea-surface temperatures fluctuated between glacial values (∼2 °C) and interglacial values (∼10 °C) on a subdecadal to multidecadal time scale. This period of extreme instability preceded the more stable and warm Holocene, starting ca. 11,500 calendar yr B.P. These results are consistent with modeling experiments that link the Younger Dryas climate changes to fluctuations of the North Atlantic thermohaline circulation.