The Antarctic ice sheet is buttressed by floating ice shelves that calve icebergs along large fractures called rifts. We report the first-ever seismic recording of a multiple-kilometer rift propagation event located in Pine Island Glacier Ice Shelf. The rift grew 10.5 km at a speed of 34.8 m/s, the fastest known ice fracture at this scale. We simulate ocean-coupled rift propagation and find that hydrodynamics control rupture velocities. During rift propagation, ocean water flows into the rift at a rate of at least 2300 m3/s and causes mixing in the subshelf cavity. Our observations support the hypotheses that large ice shelf rift propagation events are brittle, hydrodynamically limited, and exhibit sensitive coupling with the surrounding ocean.