CoS₂ is considered to be a promising electrocatalyst for hydrogen evolution reaction (HER). However, its further widespread applications are hampered by the unsatisfactory activity due to relatively high chemisorption energy for hydrogen atom. Herein, theoretical predictions of first-principles calculations reveal that the introduction of a Cl-terminated MXenes-Ti₃CNCl₂ can significantly reduce the HER potential of CoS₂-based materials and the Ti₃CNCl₂@CoS₂ core–shell nanostructure has Gibbs free energy of hydrogen adsorption (|ΔG_H|) close to zero, much lower than that of the pristine CoS₂ and Ti₃CNCl₂. Inspired by the theoretical predictions, we have successfully fabricated a unique Ti₃CNCl₂@CoS₂ core–shell nanostructure by ingeniously coupling CoS₂ with a Cl-terminated MXenes-Ti₃CNCl₂. Interface-charge transfer between CoS₂ and Ti₃CNCl₂ results in a higher degree of electronic localization and a formation of chemical bonding. Thus, the Ti₃CNCl₂@CoS₂ core–shell nanostructure achieves a significant enhancement in HER activity compared to pristine CoS₂ and Ti₃CNCl₂. Theoretical calculations further confirm that the partial density of states of CoS₂ after hybridization becomes more non-localized, and easier to interact with hydrogen ions, thus boosting HER performance. In this work, the success of oriented experimental fabrication of high-efficiency Ti₃CNCl₂@CoS₂ electrocatalysts guided by theoretical predictions provides a powerful lead for the further strategic design and fabrication of efficient HER electrocatalysts.