The development of advanced catalysts for efficient electrochemical energy conversion technologies to alleviate the reliance on fossil fuels has attracted considerable interest in the last decades. Insight into the roles of reactive sites in nanomaterials is significant for understanding and implementing the design principles of nanocatalysts. Recently, the essential role of defects, including vacancies, reconstructed defects, and doped non-metal (or metal)-defect-based motifs, have been widely demonstrated to promote the diverse electrochemical processes (e.g., O2 [or CO2] reduction reactions and H2 [or O2] evolution reactions). Nevertheless, the in-depth exploration of the underlying defect electrocatalytic mechanism is still in its infancy. This review summarizes the state-of-the-art defect engineering strategies for designing highly efficient electrochemical nanocatalysts with special emphasis on the correlation between defect structures and electrocatalytic properties. Finally, some perspectives on the challenges and future research directions in this promising area are presented.