Hydrogen evolution reaction (HER) plays a key role in combating an ever-growing imbalance and adverse climatic issues. Thus far, Pt-based catalysts are treated as “the Holy Grail” for HER and none have surpassed them. However, as the reaction sites are present only on the catalyst surface, Pt utilization is severely hampered. Scaling down the catalyst nanoparticles is a vital approach to maximize the Pt utilization. To address this issue, we fabricated PtxPdy (x/y = 1/2, 1/1, and 2/1) bimetallic nanocatalysts (NCs) downsized up to ∼1.2 nm with ultralow metal loading (∼1−3 wt %) and remarkable catalytic performance via a deposition−precipitation method on a carbon support. In the optimum case, the NC with a x/y ratio of 1/1 (i.e., Pt1Pd1) exhibits an outstanding mass activity (MA) of 9.57 A mgPt−1 at 0.05 V versus normal hydrogen electrode in acidic medium (0.5 M H2SO4) in HER. This result is 7.4-times enhanced compared to that of the Pt/C NC. Moreover, the Pt1Pd1 NC retains 81% of its MA (7.74 A mgPt−1) when operated in the chronoamperometric stability test (while 39% decay in MA is observed for the Pt/ C NC) up to 12 h. By screening the results of electron microscopy, X-ray spectroscopy, and electrochemical analysis, we revealed that such a substantial improvement is resulted from the decoration of the Pt trimer with high surface coverage and local structure ordering in the Pd crystal. Results of PtxPdy with different atomic Pt sizes are compared as a reference for rationalizing the development of ultralow-noble metal loading PtPd NCs in HER.