Synchronous acceleration and thinning of southeast (SE) Greenland glaciers during the early 2000s was the main contributor that resulted in the doubling of annual discharge from the ice sheet. We show that this acceleration was followed by a synchronized and widespread slowdown of the same glaciers, in many cases associated with a decrease in thinning rates, and we propose that ice sheet–ocean interactions are the first-order regional control on these recent mass changes. Sea surface temperature and mooring data show that the preceding dynamic thinning coincides with a brief decline in the cold East Greenland Coastal Current (EGCC) and East Greenland Current. We suggest this decline was partly induced by a reduction in ice sheet runoff, which allowed warm water from the Irminger Current to reach the SE Greenland coast. A restrengthening of the cold waters coincides with the glaciers' subsequent slowdown. We argue that this warming and subsequent cooling of the coastal waters was the cause of the glaciers' dynamic changes. We further suggest that the restrengthening of the EGCC resulted in part from cold water input by increased glacier calving during the speedup and increased ice sheet runoff. We hypothesize that the main mechanism for ice sheet mass loss in SE Greenland is highly sensitive to ocean conditions and is likely subject to negative feedback mechanisms.