Marine heatwaves (MHWs) are discrete, unusually warm-water events which can have devastating ecological impacts. In 2011, Western Australia experienced an extreme MHW, affecting >2000 km of coastline for >10 weeks. During the MHW temperatures exceeded the physiological threshold for net growth (similar to 23 degrees C) for kelp (Ecklonia radiata) along large tracts of coastline. Kelp went locally extinct across 100 km of its northern (warm) distribution. In total, an estimated 43% of the kelp along the west coast perished, and widespread shifts in species distributions were seen across seaweeds, invertebrates and fish. With the loss of kelp, turf algae expanded rapidly and now cover many reefs previously dominated by kelp. The changes in ecosystem structure led to blocking of kelp recruitment by expansive turfs and elevated herbivory from increased populations of warm-water fishes-feedback processes that prevent the recovery of kelp forests. Water temperature has long returned to pre-MHW levels, yet today, 8 years after the event, the kelp forests have not recovered. This supports initial concerns that the transformation to turf reefs represents a persistent change to a turf-dominated state. MHWs are a manifestation of ocean warming; they are being recorded with increasing frequency in all oceans, and these extreme events are set to shape our future marine ecosystems.