Photodynamic　therapy　(PDT)　has　attracted　attention　for　its　potential　for　tumor　destruction.　We　herein　report　a　pH-responsive　photosensitizer,　synthesized　by　conjugating　zinc　phthalocyanine　(ZnPc)　with　2,4,6-tris(N,N-dimethylaminomethyl)　phenoxy　(TAP),　which　exhibited　high　phototoxicity　at　pH　6.5　(slightly　acidic　condition　as　in　the　extracellular　tumor　microenvironment)　but　no　obvious　phototoxicity　at　physiological　pH　7.4.　This　pH-responsive　photosensitizer　specifically　destroyed　mouse　mammary　carcinoma　cells　4T1　with　an　IC50　of　0.20　μM　under　a　relatively　low　light　dosage　(2.5　J　cm−2).　The　subsequent　in　vivo　studies　using　4T1-bearing　mice　demonstrated　that　this　tumor-targeting　photosensitizer　ZnPc(TAP)4　not　only　ablated　tumor　cells　photodynamically　but　also　presented　clear　fluorescence　cell　imaging　of　tumor　sites.　These　findings　suggest　that　such　a　tumor-pH-responsive　photosensitizer　based　on　phthalocyanine　may　open　up　a　new　avenue　for　tumor-targeted　and　image-guided　cancer　theranostics　in　PDT.　©　The　Royal　Society　of　Chemistry.