The development of low-cost, stable, and robust non-noble metal catalysts for water oxidation is a pivotal challenge for sustainable hydrogen production through electrocatalytic water splitting. Currently, such catalysts suffer from high overpotential and sluggish kinetics in oxygen evolution reactions (OERs). Herein, we report a "continuous" single-crystal honeycomb-like MXene/NiFeP_x–N-doped carbon (NC) heterostructure, in which ultrasmall NiFeP_x nanoparticles (NPs) encapsulated in the NC are tightly anchored on a layered MXene. Interestingly, this MXene/NiFeP_x–NC delivers outstanding OER catalytic performance, which stems from "continuous" single-crystal characteristics, abundant active sites derived from the ultrasmall NiFeP_x NPs, and the stable honeycomb-like heterostructure with an open structure. The experimental results are rationalized theoretically (by density functional theory (DFT) calculations), which suggests that it is the unique MXene/NiFeP_x–NC heterostructure that promotes the sluggish OER, thereby enabling superior durability and excellent activity with an ultralow overpotential of 240 mV at a current density of 10 mA·cm⁻².