The rational design of theranostic nanoparticles exhibiting synergistic turn-on of therapeutic potency and enhanced diagnostic imaging in response to tumor milieu is critical for efficient personalized cancer chemotherapy. We herein fabricate self-reporting theranostic drug nanocarriers based on hyperbranched polyprodrug amphiphiles (hPAs) consisting of hyperbranched cores conjugated with reduction-activatable camptothecin prodrugs and magnetic resonance (MR) imaging contrast agent (Gd complex), and hydrophilic coronas functionalized with guanidine residues. Upon cellular internalization, reductive milieu-actuated release of anticancer drug in the active form, activation of therapeutic efficacy (>70-fold enhancement in cytotoxicity), and turn-on of MR imaging (similar to 9.6-fold increase in T-1 relaxivity) were simultaneously achieved in the simulated cytosol milieu. In addition, guanidine-decorated hPAs exhibited extended blood circulation with a half-life up to similar to 9.8 h and excellent tumor cell penetration potency. The hyperbranched chain topology thus provides a novel theranostic polyprodrug platform for synergistic imaging/chemotherapy and enhanced tumor uptake.