Immobilizing metal nanoparticles on a support is crucial for catalysts' stability and spatial distribution. MXenes are promising substrates for in situ growth engineering of various electrocatalysts owing to their merits. A stronger binding capacity can be achieved between the in situ -fabricated catalysts and MXenes compared to a common physical combination. Thus, synergistically utilizing morphology modulation, composition optimization, and the interfacial interaction between metal catalysts and supports will maximize the electrocatalytic activity. However, most reported in situ -formed catalysts on MXenes result in solid 0D nanoparticles and in situ growth of nanoalloy catalysts on MXenes with a precisely controlled morphology is still lacking. Herein, nanodendritic PdNi alloys are in situ grown on nitrogen-doped V 2 CT x , serving as efficient electrocatalysts toward the hydrogen evolution reaction (HER). Thanks to the synergistic effect of the unique nanodendritic structure of PdNi, the merits of N-TBA-V 2 CT x nanosheets, and the strong metal-support interaction between the PdNi and the N-TBA-V 2 CT x support, the in situ -formed Pd 58 Ni 42 /N-TBA-V 2 CT x electrocatalyst shows excellent HER performance with an ultralow overpotential of 44.1 mV to achieve 10 mA cm -2 and a lowest Tafel slope of 39.4 mV dec -1 , which outperforms Pd 58 Ni 42 /TBA-V 2 CT x , Pd 58 Ni 42 , and Pd/C. Remarkably, the Pd 58 Ni 42 /N-TBA-V 2 CT x catalyst can maintain 92.3% of its initial activity even after 50 h of continuous operation.