Radiotherapy is highly effective against solid tumor with high doses of radiation. The use of gold nanoparticles as radiosensitizers is an effective way to boost the killing efficacy while drastically limiting the received dose and reducing the possible damage to normal tissues. Herein, we designed aggregation-induced emission gold clustoluminogens (AIE-Au) to achieve efficient low-dose X-ray-induced photodynamic therapy with negligible side effects. The aggregations of glutathione-protected gold clusters (GCs) assembled by cationic polymer enhanced X-ray-excited optical luminescence by 5.2-fold. Under low-dose X-ray irradiation, AIE-Au strongly absorbed X-rays and efficiently generated hydroxyl radicals, which enhanced the radiotherapy effect. Additionally, AIE-Au converted X-rays to optical luminescence, which excited the conjugated photosensitizers, resulting in a photodynamic therapy effect. The in vitro and in vivo experiments demonstrated that AIE-Au clustoluminogens effectively triggered the generation of reactive oxygen species with an order-of-magnitude reduction in the X-ray dose, enabling highly effective cancer treatment via the unique X-PDT mechanism.