Transition-metal phosphides (TMPs) have emerged as a fascinating class of narrow-gap semiconductors and electrocatalysts. However, they are intrinsic nonlayered materials that cannot be delaminated into two-dimensional (2D) sheets. Here, we demonstrate a general bottom-up topochemical strategy to synthesize a series of 2D TMPs (e.g. Co₂P, Ni₁₂P₅, and Co_xFe_2−xP) by using phosphorene sheets as the phosphorus precursors and 2D templates. Notably, 2D Co₂P is a p-type semiconductor, with a hole mobility of 20.8 cm² V⁻¹ s⁻¹ at 300 K in field-effect transistors. It also behaves as a promising electrocatalyst for the oxygen evolution reaction (OER), thanks to the charge-transport modulation and improved surface exposure. In particular, iron-doped Co₂P (i.e. Co_1.5Fe_0.5P) delivers a low overpotential of only 278 mV at a current density of 10 mA cm⁻² that outperforms the commercial Ir/C benchmark (304 mV).