Single-atom catalysts (SACs), renowned for their maximized atomic utilization, tunable coordination environments, and unique electronic structures, are critical to energy conversion and storage. However, obstacles to their practical performance include atomic agglomeration (caused by high surface free energy) and active site passivation (due to overly strong metal-support chemical bonds). Single-atom nano-islands (SANIs) catalysts, characterized by confined spaces and innovative structural designs, have better electrocatalytic activity and stability. This review comprehensively analyzes recent advancements in SANIs catalysts, highlighting their contributions to catalytic activity, stability, structural optimization, and selectivity. We systematically summarize the design principles and strategies for SANIs electrocatalysts by focusing on material selection, metal-support interactions, and coordination structures. Finally, the challenges and opportunities associated with SANIs catalysts to promote their development in heterogeneous catalysis and accelerate their transition into industrial applications are discussed.