Defect engineering has become a promising approach to improve the performance of hydrogen evolution reaction (HER) catalysts. Non-noble transition metal-based catalysts (TMCs) have shown significant promise as effective alternatives to traditional platinum-group catalysts, attracting considerable attention. However, the industrial application of TMCs in electrocatalytic hydrogen production necessitates further optimization to boost both catalytic activity and stability. This review comprehensively examines the types, fabrication methods, and characterization techniques of various defects that enhance catalytic HER activity. Key advancements include optimizing defect concentration and distribution, coupling heteroatoms with vacancies, and leveraging the synergy between bond lengths and defects. In-depth discussions highlight the electronic structure and catalytic mechanisms elucidated through in-situ characterization and density functional theory calculations. Additionally, future directions are identified, exploring novel defect types, emphasizing precision synthesis methods, industrial-scale preparation techniques, and strategies to enhance structural stability and understanding the in-depth catalytic mechanism. This review aims to inspire further research and development in defect-engineered HER catalysts, providing pathways for high efficiency and cost-effectiveness in hydrogen production.