2D MXene nanosheets are regarded as promising cathode catalysts towards the hydrogen evolution reaction (HER), while their overall electrocatalytic ability still needs to be optimized before the practical application. In this contribution, we put forward a controllable assembly approach for constructing a heterostructured nanoarchitecture made from ultrafine MoS₂ quantum dot-decorated Ti₃C₂T_x MXene nanosheets (MQDs/Ti₃C₂T_x). Benefiting from the remarkable structural features of the ultrathin catalyst layers, homogeneous MoS₂ distribution, optimized electronic structure, and good electron conductivity, the resulting MQD/Ti₃C₂T_x catalyst exhibits superior HER properties with a low onset potential of 66 mV, a small Tafel slope of 74 mV dec⁻¹, and a long lifespan, in contrast to the insufficient HER ability for bare MoS₂ quantum dots and Ti₃C₂T_x itself. DFT calculations further disclose the optimized band structure of the MQDs/Ti₃C₂T_x model with numerous diverse active sites, which play fundamental roles in boosting the HER kinetics.