Polymer-based fluorescent nanomaterials have proven to universally image various tumors based on their extremely sharp responsiveness to pH change. Such a property has never been realized in supramolecular systems. We herein design a small molecule (DPP-thiophene-4) that is composed of a diketopyrrolopyrrole (DPP) core and two alkyl chains terminated with quaternary ammonium. DPP-thiophene-4 can self-assemble into a nonfluorescent nanoassembly when the pH is >7.0 but reversibly disassembles back to fluorescent monomers when the pH is <6.8. Meanwhile, its fluorescence emission increases by 10-fold within a 0.2 pH unit change. Such a fluorogenic nanoassembly can precisely differentiate a number of malignant tumors among normal tissues in vivo due to the slight acidity within tumor microenvironments. Further the nanoassembly shows satisfactory biocompatibility and an effective clearance from the body. Overall, this supramolecular fluorogenic nanoassembly exhibits an immense potential for realizing broad range tumor diagnosis.