A green, facile, one-pot synthesis of well-defined Au NPs@POM–GNSs tricomponent nanohybrids is reported (POM stands for polyoxometalate and GNSs for graphene nanosheets). The synthesis is convenient, rapid and environmentally friendly. The POMs serve as both reducing, encapsulating molecules, and bridging molecules; this avoids the introduction of other organic toxic molecules. Characterization using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy analysis is performed, and the structure of the prepared nanohybrids of Au NPs@POM–GNSs is verified. Most importantly, the amperometric measurements show the Au NPs@POM–GNSs nanohybrids have high catalytic activity with good sensitivity, good long-term stability, wide linear range, low detection limit, and fast response towards H2O2 detection for application as an enzyme-free biosensor. Transformation of the POMs during H2O2 detection does not affect the catalytic activities of the nanohybrids. Thus, the synergistic effect of Au NPs and GNSs in the nanohybrids leads to the enhanced catalytic property.