Record-high air temperatures were observed over Greenland in the summer of 2019 and melting of the northern Greenland Ice Sheet was particularly extensive. Here we show, through direct measurements, that near surface ocean temperatures in Sherard Osborn Fjord, northern Greenland, reached 4 °C in August 2019, while in the neighboring Petermann Fjord, they never exceeded 0 °C. We show that this disparity in temperature between the two fjords occurred because thick multi-year sea ice at the entrance of Sherard Osborn Fjord trapped the surface waters inside the fjord, which led to the formation of a warm and fresh surface layer. These results suggest that the presence of multi-year sea ice increases the sensitivity of Greenland fjords abutting the Arctic Ocean to climate warming, with potential consequences for the long-term stability of the northern sector of the Greenland Ice Sheet.