The development of effective and inexpensive hydrogen evolution reaction (HER) electrocatalysts for future renewable energy systems is highly desired. Platinum-based materials are the most active electrocatalysts for catalyzing HER, but reducing the use of Pt is required because of the high price and scarcity of Pt. Here, we achieve pseudo-atomic-scale dispersion of Pt, i.e. individual atoms or subnanometer clusters, on the sidewalls of single-walled carbon nanotubes (SWNTs) with a simple and readily upscalable electroplating deposition method. These SWNTs activated with an ultralow amount of Pt exhibit activity similar to that of commercial Pt/C with a notably higher (∼66–333-fold) Pt loading for catalyzing the HER under the acidic conditions required in proton exchange membrane technology. These catalysts resemble pseudo-atomic-scale Pt systems which are mainly composed of a few to tens of Pt atoms dispersed on the sidewalls of the SWNTs. The Pt loading is only 0.19–0.75 atom % at the electrode surface, and characteristic peaks for Pt cyclic voltammograms are undetectable. The atomic dispersion increases the portion of the surface active-atom sites, and therefore, notably lower Pt loading is needed to attain a high catalytic activity. Density functional theory (DFT) calculations suggest higher ability for SWNTs, in comparison to graphene, as a catalyst support for immobilizing Pt atoms, thus providing an atomic dispersion. Moreover, a high HER activity for the SWNTs activated with Pt atoms, similar to that of bulk Pt, is predicted.