The single largest-known mass wasting deposit has been identified at the Cretaceous-Paleogene (K-Pg) boundary in the deep-water Gulf of Mexico, in 31 industry-drilled wells and on seismic data, corresponding to the “MCU” (middle Cretaceous unconformity) horizon. The deposit has an average thickness of 10–20 m on the upper slope and 90–200 m on the lower slope and basin floor, and is on an unconformity that represents 9 m.y. to 85 m.y. The deposit contains the distinctive association of lithic fragments, impact-derived material, and reworked microfossils (i.e., the Cretaceous-Tertiary boundary “cocktail”) associated with the Chicxulub impact, and is predominantly composed of graded pelagic carbonates. These new findings substantiate widespread slope failure induced by the Chicxulub impact and provide further evidence of a single impact coincident with the K-Pg mass extinction.