Utilizing supported single atoms as catalysts presents an opportunity to reduce the usage of critical raw materials such as platinum, which are essential for electrochemical reactions such as hydrogen oxidation reaction (HOR). Herein, we describe the synthesis of a Pt single electrocatalyst inside single-walled carbon nanotubes (SWCNTs) via a redox reaction. Characterizations via electron microscopy, X-ray photoelectron microscopy, and X-ray absorption spectroscopy show the single-atom nature of the Pt. The electrochemical behavior of the sample to hydrogen and oxygen was investigated using the advanced floating electrode technique, which minimizes mass transport limitations and gives a thorough insight into the activity of the electrocatalyst. The single-atom samples showed higher HOR activity than state-of-the-art 30% Pt/C while almost no oxygen reduction reaction activity in the proton exchange membrane fuel cell operating range. The selective activity toward HOR arose as the main fingerprint of the catalyst confinement in the SWCNTs.