Within the past 450000 yr, a section of ocean crust ∼4 by 5 km was removed from the eastern wall of the Mid-Atlantic rift valley in a massive rockslide that disintegrated to form a debris avalanche. Slope failure probably occurred along preexisting west-dipping faults where hydrothermal metamorphism is inferred to have weakened the crust. The debris avalanche flowed more than 11 km into the rift valley and deposited a debris wedge with a volume of ∼19 x 109 m3. Run-up and rebound features occur where the leading edge of the avalanche encountered a basement ridge, and they suggest rapid transport in an event that lasted only a few minutes. This is the first known occurrence of such large-scale mass wasting along the tectonically active rift valley of a mid-ocean ridge.