Background: Secondary-iron overload, alloimmunization, and increased risk of infections are common complications in β-thalassemia major (BTM) patients. Tregs and myeloid-derived suppressor cells (MDSCs) play an essential role in preventing excessive immune responses. This research aimed to investigate the interaction between Tregs and MDSCs in BTM patients and their relation with disease severity.Methods: This case-control study included 26 patients with BTM and 23 healthy age- and sex-matched controls. All patients were investigated for complete blood picture, serum ferritin, and flow cytometric analysis of peripheral blood to detect Tregs, MDSCs, and MDSC subsets.Results: A significant increase was observed in the frequencies of Tregs and MDSCs, particularly MO-MDSCs, in the patients compared with the controls. These cells also showed direct relations with ferritin and TLC and an inverse association with hemoglobin. Furthermore, a positive correlation was seen between Tregs and each of the total MDSCs and MO-MDSCs. Conclusion: Our findings highlight the role Tregs and MDSCs cooperatively plays in the BTM and their importance in suppressing the high activity of the immune system found in those patients due to repeated blood transfusions and antigenic stimulation.