Purinergic signaling modulates immune function and is involved in the immunopathogenesis of several viral infections. This study aimed to investigate alterations in purinergic pathways in coronavirus disease 2019 (COVID-19) patients. Mild and severe COVID-19 patients had lower extracellular adenosine triphosphate and adenosine levels, and higher cytokines than healthy controls. Mild COVID-19 patients presented lower frequencies of CD4 + CD25 + CD39 + (activated/memory regulatory T cell [mTreg]) and increased frequencies of high-differentiated (CD27 − CD28 − ) CD8 + T cells compared with healthy controls. Severe COVID-19 patients also showed higher frequencies of CD4 + CD39 + , CD4 + CD25 − CD39 + (memory T effector cell), and high-differentiated CD8 + T cells (CD27 − CD28 − ), and diminished frequencies of CD4 + CD73 + , CD4 + CD25 + CD39 + mTreg cell, CD8 + CD73 + , and low-differentiated CD8 + T cells (CD27 + CD28 + ) in the blood in relation to mild COVID-19 patients and controls. Moreover, severe COVID-19 patients presented higher expression of PD-1 on low-differentiated CD8 + T cells. Both severe and mild COVID-19 patients presented higher frequencies of CD4 + Annexin-V + and CD8 + Annexin-V + T cells, indicating increased T-cell apoptosis. Plasma samples collected from severe COVID-19 patients were able to decrease the expression of CD73 on CD4 + and CD8 + T cells of a healthy donor. Interestingly, the in vitro incubation of peripheral blood mononuclear cell from severe COVID-19 patients with adenosine