Reactive oxygen species (ROS)-induced apoptosis is a promising treatment strategy for malignant neoplasms. However, current systems are highly dependent on oxygen status and/or external stimuli to generate ROS, which greatly limit their therapeutic efficacy particularly in hypoxic tumors. Herein, we develop a biomimetic nanoflower based on self-assembly of nanozymes that can catalyze a cascade of intracellular biochemical reactions to produce ROS in both normoxic and hypoxic conditions without any external stimuli. In our formulation, PtCo nanoparticles are firstly synthesized and used to direct the growth of MnO₂. By adjusting the ratio of reactants, highly-ordered MnO₂@PtCo nanoflowers with excellent catalytic efficiency are obtained, where PtCo behaves as oxidase mimic and MnO₂ functions as catalase mimic. In this way, the well-defined MnO₂@PtCo nanoflowers not only can relieve hypoxic condition but also induce cell apoptosis significantly through ROS-mediated mechanism, thereby resulting in remarkable and specific inhibition of tumor growth.