Oxidative　stress　and　chronic　inflammation　have　been　implicated　in　the　pathophysiology　of　metabolic　diseases,　including　diabetes　mellitus　(DM),　metabolic　syndrome　(MS),　fatty　liver　(FL),　atherosclerosis　(AS),　and　obesity.　Molecular　hydrogen　(H-2)　has　long　been　considered　a　physiologically　inert　gas.　In　the　last　two　decades,　accumulating　evidence　from　pre-clinical　and　clinical　studies　has　indicated　that　H-2　may　act　as　an　antioxidant　to　exert　therapeutic　and　preventive　effects　on　various　disorders,　including　metabolic　diseases.　However,　the　mechanisms　underlying　the　action　of　H-2　remain　unclear.　The　purpose　of　this　review　was　to　(1)　provide　an　overview　of　the　current　research　on　the　potential　effects　of　H-2　on　metabolic　diseases;　(2)　discuss　the　possible　mechanisms　underlying　these　effects,　including　the　canonical　anti-oxidative,　anti-inflammatory,　and　anti-apoptotic　effects,　as　well　as　suppression　of　ER　stress,　activation　of　autophagy,　improvement　of　mitochondrial　function,　regulation　of　gut　microbiota,　and　other　possible　mechanisms.　The　potential　target　molecules　of　H-2　will　also　be　discussed.　With　more　high-quality　clinical　trials　and　in-depth　mechanism　research,　it　is　believed　that　H-2　will　eventually　be　applied　to　clinical　practice　in　the　future,　to　benefit　more　patients　with　metabolic　disease.