The Preboreal oscillation (PBO) has been attributed to increased meltwater, but the source of the meltwater and causative mechanism of the PBO has remained elusive. Here we attribute the source to a massive meltwater discharge event from an abrupt drainage of glacial Lake Agassiz, Canada, via the Mackenzie River into the Arctic Ocean. A maximum volume of 21,000 km 3 was discharged over a 1.5-3 yr period with a peak discharge of 0.500 Sverdrups (Sv), equivalent to a 6 m rise in the Arctic Ocean (or 0.062 m rise in global sea level). The flood occurred at about 11,335 cal yr BP, and was followed by a ̃0.042 Sv flow until 10,750 cal yr BP when the southern outlet of Lake Agassiz reopened and diverted drainage to the Mississippi River system. We estimate that only 2-4% of the flood water would have frozen into sea ice within the Beaufort region, but coupled with increased river ice production during winter, and thicker pack ice growth throughout the Arctic Ocean, a thicker, longer lasting and more extensive pack ice may have been flushed through Fram Strait. The thicker and more extensive pack ice, and freshened sea surface, may have triggered the PBO by increasing albedo, and generating a low salinity anomaly upon melting in the North Atlantic, thus decreasing the formation of North Atlantic Deep Water.