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The Earth’s surface is constantly being recycled by plate tectonics. Subduction of oceanic lithosphere and delamination of continental lithosphere constitute the two most important mechanisms by which the Earth’s lithosphere is recycled into the mantle. Delamination or detachment in continental regions typically occurs below mountain belts due to a weight excess of overthickened lithospheric mantle, which detaches from overlying lighter crust, aided by the existence of weak layers within the continental lithosphere. Oceanic lithosphere is classically pictured as a rigid plate with a strong core that does not allow for delamination to occur. Here, we propose that active delamination of oceanic lithosphere occurs offshore Southwest Iberia. The process is assisted by the existence of a lithospheric serpentinized layer that allows the lower part of the lithosphere to decouple from the overlying crust. Tomography images reveal a sub-lithospheric high-velocity anomaly below this region, which we interpret as a delaminating block of old oceanic lithosphere. We present numerical models showing that for a geological setting mimicking offshore Southwest Iberia delamination of oceanic lithosphere is possible and may herald subduction initiation, which is a long-unsolved problem in the theory of plate tectonics. We further propose that such oceanic delamination is responsible for the highest-magnitude earthquakes in Europe, including the M8.5-8.7 Great Lisbon Earthquake of 1755 and the M7.9 San Vincente earthquake of 1969. In particular, our numerical models, in combination with calculations on seismic potential, provide a solution for the instrumentally recorded 1969 event below the flat Horseshoe abyssal plain, away from mapped tectonics faults. Delamination of old oceanic lithosphere near passive margins constitutes a new class of subduction initiation mechanisms, with fundamental implications for the dynamics of the Wilson cycle.