The fundamental relationship between structure and properties, which is called “structure-property”, plays a vital role in the rational designing of high-performance catalysts for diverse electrocatalytic applications. Low-dimensional (LD) nanomaterials, including 0D, 1D, 2D materials, combined with low-nuclearity metal atoms, ranging from single atoms to subnanometer clusters, are currently emerging as rising star nanoarchitectures for heterogeneous catalysis due to their well-defined active sites and unbeatable metal utilization efficiencies. In this work, a comprehensive experimental and theoretical review is provided on the recent development of single atom and atomic cluster-decorated LD platforms towards some typical clean energy reactions, such as water-splitting, nitrogen fixation, and carbon dioxide reduction reactions. The upmost attractive structural properties, advanced characterization techniques, and theoretical principles of these low-nuclearity electrocatalysts as well as their applications in key electrochemical energy devices are also elegantly discussed. © 2022 The Authors. Advanced Energy Materials published by Wiley-VCH GmbH.