Rainfall was witnessed for the first time at the highest area of the Greenland Ice Sheet on 14 August, 2021. The thermodynamic mechanisms supporting the rainfall are revealed by ERA5 reanalysis, in-situ and satellite data. We find that a strong southward intrusion of the polar vortex favored the maintenance of a deep cyclone over Baffin Island and an amplification of anticyclonic circulation over the southeastern ice sheet, which pumped warm and moist air toward Greenland from anomalously warm waters south of Greenland. Across a wide swath of the ice sheet, atmospheric uplift maintained above-melting and rainfall conditions via condensation and enhanced downward infrared irradiance. Without the low-level liquid clouds, the spatial extent and duration of the rainfall would have been smaller. Over the ice sheet topographic summit, the air temperature from the ground to 250 hPa level was ~2 °C higher than the previous record set on 12 July, 2012. Such events may occur more frequently with the decreased temperature contrast between the Arctic and the mid-latitude regions that drives highly amplified jet streams. Thus, this extreme event serves as a harbinger of a more likely wet surface condition across all elevations of the ice sheet.