Grain boundary (GB), as a type of two-dimensional defect in nanocrystals, has recently been identified as a kind of highly active site for various electrocatalytic reactions, such as hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), nitrogen/nitrate reduction reaction (NRR/NO₃RR), CO₂ reduction reaction (CO₂RR), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and alcohol oxidation reaction (methanol/ethanol oxidation reaction (MOR/EOR)). However, there is still a lack of comprehensive reviews examining how GB engineering affects catalytic activity. In this review, we summarize recent advances in GB-engineered metal-based nanomaterials, highlight the remarkable effects of GB, and discuss the structure–performance relationship of GB-rich nanocrystals in electrocatalytic reactions. First, we introduce the synthesis and characterization of GB-engineered nanocatalysts. Next, we thoroughly discuss the relationship between the structure and performance of GB-engineered nanocatalysts. Finally, we outline the challenges and opportunities in this rapidly evolving field, i.e., GB engineering of nanocatalysts.