BACKGROUND: Thymosin alpha 1 (Talpha1) had been used in the treatment of viral infections as an immune response modifier for many years. However, clinical benefits and the mechanism of Talpha1 treatment for COVID-19 patients are still unclear. METHODS: We retrospectively reviewed the clinical outcomes of 76 severe COVID-19 cases admitted to 2 hospitals in Wuhan, China, from December 2019 to March 2020. The thymus output in peripheral blood mononuclear cells from COVID-19 patients was measured by T-cell receptor excision circles (TRECs). The levels of T-cell exhaustion markers programmed death-1 (PD-1) and T-cell immunoglobulin and mucin domain protein 3 (Tim-3) on CD8+ T cells were detected by flow cytometry. RESULTS: Compared with the untreated group, Talpha1 treatment significantly reduced the mortality of severe COVID-19 patients (11.11% vs 30.00%, P = .044). Talpha1 enhanced blood T-cell numbers in COVID-19 patients with severe lymphocytopenia. Under such conditions, Talpha1 also successfully restored CD8+ and CD4+ T-cell numbers in elderly patients. Meanwhile, Talpha1 reduced PD-1 and Tim-3 expression on CD8+ T cells from severe COVID-19 patients compared with untreated cases. It is of note that restoration of lymphocytopenia and acute exhaustion of T cells were roughly parallel to the rise of TRECs. CONCLUSIONS: Talpha1 treatment significantly reduced mortality of severe COVID-19 patients. COVID-19 patients with counts of CD8+ T cells or CD4+ T cells in circulation less than 400/muL or 650/muL, respectively, gained more benefits from Talpha1. Talpha1 reversed T-cell exhaustion and recovered immune reconstitution through promoting thymus output during severe acute respiratory syndrome-coronavirus 2 infection.