The exploration of a facile strategy to synthesize porous ultrathin nanosheets of non-layered materials, especially with exposed reactive facets, as highly efficient electrocatalysts for the hydrogen evolution reaction (HER), remains challenging. Herein we demonstrate a chemical transformation strategy to synthesize porous CoP ultrathin nanosheets with sub-1.1 nm thickness and exposed {200} facets via phosphidation of Co₃O₄ precursors. The resultant samples exhibit outstanding electrochemical HER performance: a low overpotential (only 56 and 131 mV are required for current densities of 10 and 100 mA cm⁻², respectively), a small Tafel slope of 44 mV per decade, a good stability of over 20 h, and a high mass activity of 151 A g⁻¹ at an overpotential of 100 mV. The latter is about 80 times higher than that of CoP nanoparticles. Experimental data and density functional theory calculations reveal that a high proportion of reactive {200} facets, high utilization efficiency of active sites, metallic nature, appropriate structural disorder, facile electron/mass transfer and rich active sites benefiting from the unique ultrathin and porous structure are the key factors for the greatly improved activity. Additionally, this facile chemical conversion strategy can be developed to a generalized method for preparing porous ultrathin nanosheets of CoSe₂ and CoS that cannot be obtained using other methods.