This study aimed to assess the efficacy of oral supplementation with L-glutathione, specifically enhanced with bioavailability boosters (namely tannin, low molecular weight chitosan, and polyethylene glycol), on endogenous reduced glutathione (GSH) levels, markers of oxidative damage, and glycemic regulation in patients with type 2 diabetes (T2D). In a randomized, double-blind, placebo-controlled trial, 250 T2D patients were enrolled, with 217 completing the study. Baseline and 180-day post-supplementation levels of GSH, oxidized glutathione (GSSG), 8-hydroxydeoxyguanosine (8-OHdG), glycated hemoglobin (HbA1c), and various metabolic parameters were measured. After 180 days, the L-glutathione group exhibited a significant increase in GSH levels (119.78%) compared to the placebo group (22.53%), with the difference between groups being 97.25% (95% CI [92.20%, 102.30%]) (p < 0.01). Significant reductions in GSSG and 8-OHdG levels were also observed, indicating decreased oxidative stress. Variations in HbA1c and insulin levels were noted, though not consistently reaching statistical significance. L-glutathione supplementation, when combined with specific bioavailability enhancers, significantly increases endogenous GSH levels and improves antioxidant defense in T2D patients. The role of L-glutathione as a therapeutic strategy offers a promising adjunct to current diabetes management practices, emphasizing the need for a comprehensive treatment approach. This approach should not only aim to achieve glycemic control but also to improve the oxidative status of patients, thereby potentially reducing the risk of long-term complications. The integration of L-glutathione supplementation into diabetes care protocols could represent a significant step forward in the quest for more effective, holistic treatment strategies. Future research, building on the foundations laid by this study, will be crucial in optimizing dosing, understanding long-term impacts, and further elucidating the mechanisms by which L-glutathione and other antioxidants can best be used to improve the health and well-being of individuals with type 2 diabetes.