Potassium-ion batteries (KIBs) are among the most promising alternatives to lithium-ion batteries (LIBs), but suffer from substantial challenges for success due to difficulties in constructing suitable electrode materials that can robustly host the large ionic radius of K⁺ ions. Herein, a desired CoP/carbon composite with ultrasmall CoP nanoparticles confined into nitrogen-doped porous carbon (CoP@NPC) is facilely constructed by a synchronous pyrolysis and phosphorization strategy. Relying on the structural merits that can offer highly electrical conductivity, reduced and effective transport pathways, abundant active sites and robust structural stability, the composite can achieve efficient K-storage including relatively higher reversible capacity, better rate capability and enhanced long cycling stability. In addition, K⁺ ions diffusion kinetics and electrochemical storage mechanism are elucidated. The present work is expected to provide a new venue for the facile design and construction of advanced transition-metal phosphides (TMPs)-based electrode materials for rechargeable KIBs.