Optical trapping has widely affected both the physical and life sciences. Past approaches to optical trapping of nanoscale objects required large optical intensities, often above their damage threshold. To achieve more than an order of magnitude reduction in the local intensity required for optical trapping, we present a self-induced back-action (SIBA) optical trap, where the trapped object has an active role in enhancing the restoring force. We demonstrate experimentally trapping of a single 50 nm polystyrene sphere using a SIBA optical trap on the basis of the transmission resonance of a nanoaperture in a metal film. SIBA optical trapping shows a striking departure from previous approaches, which we quantify by comprehensive calculations. The SIBA optical trap enables new opportunities for non-invasive immobilization of a single nanoscale object, such as a virus or a quantum dot.