Highlights4-nm-thick Pt nanosheets (Pt-NSs) are electrochemically grown on Ti substrates for hydrogen evolution reactions.Highly uniform Pt-NS surface coverage with an ultralow loading of 0.015 mgPtcm−2is achieved.99.5% catalyst savings and about 237-fold higher catalyst utilization are demonstrated.Supplementary InformationThe online version contains supplementary material available at 10.1007/s40820-023-01117-2.