Reducing　Pt　usage　in　hydrogen　evolution　reaction　(HER)　electrocatalysts　remains　a　great　challenge　to　achieve　superior　catalytic　activity.　Herein,　a　facile　hydrogen-assisted　defect-engineering　strategy　was　developed　to　create　oxygen　vacancies　on　MoO3-x　support　to　promote　Pt　dispersion　and　target　an　ultra-low　Pt　loading　towards　both　acidic　and　alkaline　HER.　The　engineered　defects　in　MoO3-x　provided　more　sites　to　accommodate　Pt,　leading　to　a　much　enhanced　total　Pt　dispersion　of　66.19%　as　compared　to　24.46%　on　defect-free　MoO3.　The　optimized　Pt/MoO3-x　exhibited　excellent　HER　electrocatalytic　performance,　with　overpotentials　as　small　as　38.9　mV　(alkaline)　and　28.1　mV　(acidic)　at　a　current　density　of　10　mA　cm−2　by　only　requiring　an　ultra-low　Pt　mass　loading　of　3.1　μg　cm−2.　Through　both　experimental　and　theoretical　investigations,　the　oxygen　vacancies　in　MoO3-x　generated　great　interactions　with　Pt　and　balanced　the　energy　barriers　for　water　dissociation　and　hydrogen　bonding,　giving　rise　to　fast　HER　kinetics.　Our　findings　demonstrated　an　effective　defect-engineering　strategy　to　achieve　ultra-low　Pt　electrocatalysts　with　superior　HER　electrocatalytic　activity　and　stability.　©　2024