Marine outlet glaciers on Greenland are retreating, yet it is unclear if the recent fast retreat will persist, and how atmosphere and ocean warming will impact future retreat. We show how a marine outlet glacier in Hardangerfjorden retreated rapidly in response to the abrupt warming following the Younger Dryas cold period (approximately 11,600 years before present). This almost 1000 m deep fjord, with several sills at 300-500 m depth, hosted a 175 km long outlet glacier at the western rim of the Scandinavian Ice Sheet. We use a dynamic ice-flow model constrained by well-dated terminal and lateral moraines to simulate the reconstructed 500-year retreat of Hardangerfjorden glacier. The model includes an idealized oceanic and atmospheric forcing based on reconstructions, but excludes the surface mass balance-elevation feedback. Our simulations show a highly episodic retreat driven by surface melt and warming fjord waters, paced by the fjord bathymetry. Warming air and ocean temperatures by 4-5 degrees C during the period of retreat result in a 125-km retreat of Hardangerfjorden glacier in 500 years. Retreat rates throughout the deglaciation vary by an order of magnitude from 50 to 2500 m a(-1), generally close to 200 m a(-1), punctuated by brief events of swift retreat exceeding 500 m a(-1), each event lasting a few decades. We show that the fastest retreat rates occur in regions of the bed with the largest retrograde slopes; ice shelf length and fjord water depth is less important. Our results have implications for modern glacial fjord settings similar to Hardangerfjorden, where high retreat rates have been observed. Our findings imply that increasing air temperatures and warming subsurface waters in Greenland fjords will continue to drive extensive retreat of marine outlet glaciers. However, the recent high retreat rates are not expected to be sustained for longer than a few decades due to constraints by the fjord bathymetry.