<p>Submarine landslides reshape seafloor geomorphology, transport sediment and carbon to the deep-sea, and can trigger tsunamis or damage valuable seabed infrastructure. Yet, submarine landslides have never been directly observed in action. Here, we present the most detailed measurements yet of an ongoing submarine landslide using high-frequency (1–10 Hz) ground motions captured by an ocean-bottom seismometer placed in the vicinity of the slide’s source region. We tracked a ~ 0.00423 km³ flank collapse in the Congo Canyon, and reveal its triggers and chronology of movement, thereby testing fundamental landslide models. A powerful turbidity current undercut the canyon wall, and landslide failure initiated at the base-of-slope. The landslide retrogressed 1.3 km upslope for 15 minutes in 1–2 min pulses, interspersed with brief downslope movement. Retrogression speeds ranged from 1.6 to 5.8 m/s, and downslope movement speeds from 0.8 to 3.3 m/s. Once the failure reached its upslope limit, the landslide transitioned into 1.5 hours of pulsed sediment transport, which fed a turbidity current along the canyon-axis. These observations show seismic monitoring can produce major advances in the understanding of submarine landslides. They challenge simple “toe-backward” and “head-forward” collapse models, revealing a hybrid, bottom-up retrogressive process that only released sediment after climbing far upslope.</p>