Alloying noble metal catalysts with early transition metals (ETMs) has shown great promise by simultaneously boosting catalytic activity and durability because of their strong electronic interactions. However, the very negative reduction potential of ETMs has posed great challenges for the synthesis of the desired alloy catalysts, not to mention the structure‐controlled synthesis. Here an autocatalytic surface reduction‐assisted strategy is reported to realize the controllable synthesis of ultrathin PtW alloy nanowires (NWs). The experimental evidence and density functional theory (DFT) calculations demonstrate that the preformed Pt NWs in the synthesis serve as the catalyst to facilitate the reduction of W^x+ species through the autocatalytic surface reduction mechanism. Using the alkaline hydrogen evolution reaction (HER) as a model reaction, the as‐synthesized PtW NWs/C catalyst shows an ultralow overpotential of 18 mV at 10 mA cm^–2 and a high mass activity of 6.13 A mg^–1_Pt at an overpotential of 100 mV, ranking it among the most active catalysts. The dual roles of alloyed W atoms are further uncovered by theoretical simulations, involving the ensemble effect for accelerating H₂O dissociation and a ligand effect for optimizing the hydrogen adsorption strength.