The end-Permian mass extinction was the most severe loss of life in Earth history, yet the role of ocean oxygen restriction remains debated. We examined rock records from South China and western Canada to track redox conditions before and after the crisis. Oxygen levels were already low prior to the extinction, and widespread seafloor anoxia developed at its peak. Afterwards, ferruginous conditions dominated, reducing nutrient availability, limiting productivity, and slowing life’s recovery.