The　development　of　highly　efficient　and　low-cost　electrocatalysts　for　hydrogen　production　through　water　splitting　is　extremely　crucial　and　challenging.　The　introduction　of　transition　metals　to　precious　metals　to　form　alloy　electrocatalysts　has　been　considered　an　effective　strategy　for　regulating　the　electronic　structure　and　catalytic　properties.　Moreover,　particle　size　reduction　is　also　a　method　for　achieving　effective　electrocatalysts.　In　this　study,　we　developed　a　simple　two-step　method　for　synthesizing　ultra-small　carbon-supported　FeRu　alloy　nanoparticles.　The　bifunctional　Fe0.05Ru0.05/XC-72　electrocatalyst　with　a　diameter　of　2.1　nm　exhibited　excellent　activity　and　durability　in　alkaline　fresh　water　and　seawater　electrolytes.　The　Fe0.05Ru0.05/XC-72　electrocatalyst　required　overpotentials　of　13,　15,　and　18　mV　at　a　current　density　of　10　mA　cm−2　in　1　mol　L−1　KOH,　1　mol　L−1　KOH　+　0.5　mol　L−1　NaCl,　and　1　mol　L−1　KOH　+　seawater,　respectively.　The　Fe0.05Ru0.05/XC-72　electrocatalyst　achieved　a　mass　activity　of　11.32　(Formula　Presented.)　at　−0.07　V　vs.　reversible　hydrogen　electrode　in　1　mol　L−1　KOH.　Moreover,　the　Fe0.05Ru0.05/XC-72　electrocatalyst　exhibited　long-term　hydrogen　evolution　reaction　(HER)　stability　over　24　h　in　1　mol　L−1　KOH.　Theoretical　calculations　indicate　that　the　FeRu　alloy　could　induce　an　electronic　effect,　leading　to　stronger　*H　adsorption　ability　and　enhancing　the　HER　performance.　This　study　presents　a　promising　approach　for　the　future　fabrication　of　excellent　Ru-based　electrocatalysts　with　high　electroactivity　and　stability.[Figure　not　available:　see　fulltext.]　©　2023,　Science　China　Press.