Black　phosphorus　(BP)　nanomaterials　have　emerged　as　rapidly　rising　stars　in　the　field　of　nanomedicine.　In　this　work,　BP　quantum　dots　(BPQDs)　are　synthesized　and　their　potential　as　photosensitizers　is　investigated　for　the　first　time.　The　BPQDs　present　good　stability　in　physiological　medium　and　no　appreciable　cytotoxicity.　More　importantly,　the　BPQDs　can　be　rapidly　eliminated　from　the　body　in　their　intact　form　via　renal　clearance　due　to　their　ultrasmall　hydrodynamic　diameter　(5.4　nm).　Both　in　vitro　and　in　vivo　studies　indicate　that　the　BPQDs　have　excellent　photodynamic　effect　under　light　irradiation　that　can　effectively　generate　reactive　oxygen　species　to　kill　cancer　cells.　The　BPQDs　thus　can　serve　as　biocompatible　and　powerful　photosensitizers　for　efficient　photodynamic　therapy.