For hydrogen generation from catalytic hydrolysis of sodium borohydride (NaBH₄), magnetically recyclable Co₂P nanosheets as highly efficient catalysts were developed in this work. Briefly, Co(OH)₂ nanosheets with an average thickness of 3.8 nm were firstly synthesized by a facile chemical precipitation, followed by controllable phosphidation for the preparation of 2D cobalt phosphides as catalysts. For characterization, a series of advanced analytical techniques were performed, including field-emission scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, N₂ adsorption–desorption measurement and magnetism analysis. The effect of the phosphidation temperature on catalyst activity was investigated to determine the optimum conditions. The thin Co₂P nanocatalyst prepared at 350 °C presented a maximum hydrogen generation rate of 6.34 L min⁻¹ g⁻¹ using 6 wt% NaBH₄–4 wt% NaOH (25 °C), which is much higher than those of similar cobalt-based catalysts and even some noble-metal catalysts reported in the literature. The apparent activation energy of the catalytic hydrolysis was measured to be 47.7 kJ mol⁻¹. The resulting 2D Co₂P nanocatalyst with ferromagnetic nature can be magnetically recycled and still retain 70% of the initial activity even after five runs.