Developing　active,　stable,　and　cost-efficient　electrocatalysts　to　replace　platinum　for　the　alkaline　hydrogen　evolution　reaction　(HER)　is　highly　desirable　yet　represents　a　great　challenge.　Here,　it　is　reported　on　a　facile　one-pot　synthesis　of　RuxNi　layered　double　hydroxides　(RuxNi-LDHs)　that　exhibit　remarkable　HER　activity　and　stability　after　an　in-situ　activation　treatment,　surpassing　most　state-of-the-art　Ru-based　catalysts　as　well　as　commercial　Ru/C　and　Pt/C　catalysts.　The　structural　and　chemical　changes　triggered　by　in-situ　activation　are　systematically　investigated,　and　the　results　clearly　show　that　the　pristine,　less-active　RuxNi-LDHs　are　transformed　into　a　highly　active　catalyst　characterized　by　raft-like,　defect-rich　Ru°　particles　decorated　on　the　surface　of　RuxNi-LDHs.　Density　functional　theory　(DFT)　calculations　reveal　that　the　defective　Ru　sites　can　effectively　optimize　the　reaction　pathway　and　lower　the　free　energies　of　the　elemental　steps　involved,　leading　to　enhanced　intrinsic　activity.　This　work　highlights　the　importance　of　the　currently　understudied　strategy　of　defect　engineering　in　boosting　the　HER　activity　of　Ru-based　catalysts　and　offers　an　effective　approach　involving　in-situ　electrochemical　activation　for　the　development　of　high-performance　alkaline　HER　catalysts.　©　2024　Wiley-VCH　GmbH.