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Maximizing the Pt utilization is important for the widescale implementation of Pt‐based hydrogen evolution reaction (HER) electrocatalysts, owing to the scarcity of Pt. Here, three‐component heterostructured HER catalysts with ultrahigh Pt mass activity in which hollow PtCu alloy nanospheres are supported on an array of WO3 on Cu foam, are reported. It has been pointed out that the use of Pt counter electrode in a three‐electrode configuration in evaluating catalysts' HER performances in acidic media carries the risk of contaminating the working electrode in previous reports. Here, the authors rationally utilize this "contaminating" to "activate" low‐HER‐activity materials, maximizing the Pt utilization. As a result, ultrahigh Pt mass activity is achieved, that is 1.35 and 10.86 A mg−1Pt at overpotentials of 20 and 100 mV, respectively, 27 and 13 times higher than those of commercial Pt/C catalysts, outperforming some state‐of‐the‐art Pt‐single‐atom catalysts. The hollow sphere structure and PtCu alloying increase the number and reactivity of active sites. Density function calculations and electrochemical experiments reveal that the synergy between WO3 and Pt is also responsible for the high HER activity where the hydrogen spillover effect triggers the Volmer–Heyrovsky mechanism and promotes the rapid removal of H* from Pt to re‐expose the active sites. [ABSTRACT FROM AUTHOR]