Phosphorene, a monolayer of bulk black phosphorus, is promising for light harvest owing to its high charge mobility and tunable direct band gap covering a broad spectral range of light. Here, via atomic-scale first-principles simulations, we report an ultrahigh activity of the hydrogen evolution reaction (HER) of phosphorene which originated from defective activation. Quantitative evaluation of the Gibbs free energy of the ad/desorption of hydrogen (H*) to/from phosphorene (Î"G) reveals that atomic vacancies and edges play a dominant role in activating the reaction. We find that the defective states, empty and well-localized around the defect core, are compensated by H∗ species. This induces a proper hydrogen interaction complying with the thermoneutral conditions of the free energy (Î"G ≈ 0) comparable to platinum. Our findings of the highly activating defective states suggest the design of nonmetal HER catalysts with structural engineering of earth-abundant phosphorus structures.