<p>Platinum clusters (Pt_<i>n</i>) are extensively used as electrocatalysts for oxygen reduction reaction (ORR) because they provide excellent performance together with a reduced Pt requirement. However, the precise synthesis and atomic-level insights into the structure-activity relationship of Pt_<i>n</i> remain a great challenge. Here, we present a combinatorial synthesis and analysis method to investigate the atomicity-activity relationships of Pt_<i>n</i> at the individual level. We employ single nanoparticle collision electrochemistry to facilitate the <i>in-situ</i> electrodeposition of a single precisely tunable Pt_<i>n</i> on the graphene quantum dot support, followed by instantaneous measurement of the intrinsic ORR activity of the resulting Pt_<i>n</i>. By relying on highly sensitive electrochemical measurements, our investigations clarify the atomicity-specific ORR activity of Pt_<i>n</i>, which is attributed to their distinct geometric and electronic structures at varying cluster sizes. Significantly, Pt_<i>n</i> with low atomicity, especially below 20, can reach extraordinarily high ORR activities due to atom-by-atom arrangement. Our work provides a simple and efficient method for investigating the atomicity-activity relationships of other nanoclusters under real reaction conditions, enabling a better design of the electrocatalysts at the atomic level.</p>