Abstract2D‐layered materials have attracted increasing attention as low‐cost supports for developing active catalysts for the hydrogen evolution reaction (HER). In addition, atomically thin Ti₃C₂T_x (MXene) nanosheets have surface termination groups (T_x: F, O, and OH), which are active sites for effective functionalization. In this work, heteroatom (boron)‐doped Ti₃C₂T_x (MXene) nanosheets are developed as an efficient solid support to host ultrasmall ruthenium (Ru) nanoparticles for electrocatalytic HER. The quantum‐mechanical first‐principles calculations and electrochemical tests reveal that the B‐doping onto 2D MXene nanosheets can largely improve the intermediate H* adsorption kinetics and reduce the charge‐transfer resistance toward the HER, leading to increased reactivity of active sites and favorable electrode kinetics. Importantly, the newly designed electrocatalyst based on Ru nanoparticles supported on B‐doped MXene (Ru@B–Ti₃C₂T_x) nanosheets shows a remarkable catalytic activity with low overpotentials of 62.9 and 276.9 mV to drive 10 and 100 mA cm⁻², respectively, for the HER, while exhibiting excellent cycling stabilities. Moreover, according to the theoretical calculations, Ru@B–Ti₃C₂T_x exhibits a near‐zero value of Gibbs free energy (ΔG_H* = 0.002 eV) for the HER. This work introduces a facile strategy to functionalize MXene for use as a solid support for efficient electrocatalysts.