In this study, we synthesized PtM (M = Mn and Co) alloy nanofascicles by a bundle of ultrathin nanowires with a diameter of ∼2.0 nm. Furthermore, the compositions of alloys were tunable. These nanofascicles exhibited significantly enhanced electrocatalytic activity due to their alloy nature, ultrathin size, and attractive morphological features. Noticeably, PtM electrocatalysts exhibited superior HER performances not only in the acidic but also in alkaline conditions when the electrocatalysts were deposited on a carbon paper. Pt₈Co demonstrates the smallest η₁₀ value of 26.1 mV under acidic conditions, which is significantly lower than that of Pt/C (63.4 mV). Under alkaline conditions, the η₁₀ of Pt₈Co is 47.2 mV, which is substantially lower than that of commercial Pt/C (187.9 mV). Pt₆Mn shows the lower η₁₀ values of 39.0 and 67.5 mV under acidic and alkaline conditions, respectively. In addition, PtM electrocatalysts exhibit excellent long-term durability under alkaline conditions.