Oxidative stress is implicated in the pathogenesis of many diseases; however, currently used antioxidants have a high toxicity that constrains administration to a narrow window of therapeutic dosage. There is a clear need for more effective and safer antioxidants. Diatomic hydrogen (H2) was proposed as a novel antioxidant that selectively reduces levels of toxic reactive-oxygen species. Recently, many studies have reported that H2 (inhaled or orally ingested, typically as approximately 0.8 mM H2-saturated water), can exert beneficial effects in diverse animal models of ischaemia–reperfusion injury, and inflammatory and neurological disease. In the clinic, oral administration of H2-saturated water is reported to improve lipid and glucose metabolism in subjects with diabetes or impaired glucose tolerance; promising results have also been obtained in reducing inflammation in haemodialysis patients and treating metabolic syndrome. These studies suggest H2 has selective antioxidant properties, and can exert antiapoptotic, anti-inflammatory and antiallergy effects. This review summarizes recent research findings and mechanisms concerning the therapeutic potential of H2.