Introduction: Electrolyzed hydrogen-rich water (EHW) is known to have suppressive effects on oxidative stress (OS). However, its benefit in type 2 diabetes mellitus (T2DM) remains unclear. This study aimed to investigate the effect of EHW on T2DM. Methods: This was a multicenter, prospective, double-blind, randomized controlled trial of 50 patients with T2DM who were assigned to the EHW or filtered water (FW) groups. The primary endpoint was changes in insulin resistance (IR) evaluated using the homeostasis model assessment of insulin resistance (HOMA-IR). OS markers such as urinary 8-hydroxy-2′-deoxyguanosine excretion (8-OHdG), plasma diacron-reactive oxygen metabolites (d-ROM), and plasma biological antioxidant potential (BAP) and other clinical data, including serum lactate concentration (lactate), were evaluated. Results: There were no significant differences in the changes in HOMA-IR between the EHW and FW groups. However, lactate levels decreased significantly in the EHW group, and this decrease was significantly correlated with a reduction in HOMA-IR, fasting plasma glucose, and fasting plasma insulin level. Serum lactate level also significantly correlated to decreased insulin bolus secretion after 90 min with glucose loading in the EHW subjects with HOMA-IR > 1.73. No EHW treatment-related adverse effects were observed. Conclusion: There were no significant effect of EHW in the change in HOMA-IR in this study; larger-scale and longer-term study are needed to verify the effects of EHW in T2DM patients. Supplementary information: The online version contains supplementary material available at 10.1007/s13340-021-00524-3.