The relatively low partial pressure of oxygen, reduced oxygen saturation, and aberrant plasma metabolites in COVID-19 may alter energy metabolism in peripheral immune cells. However, little is known regarding the immunometabolic defects of T cells in COVID-19 patients, which may contribute to the deregulated immune functions of these cells. In this study, we longitudinally characterized the metabolic profiles of resting and activated T cells from acutely infected and convalescent COVID-19 patients by flow cytometry and confirmed the metabolic profiles with a Seahorse analyzer. Non–COVID-19 and healthy subjects were enrolled as controls. We found that ex vivo T cells from acutely infected COVID-19 patients were highly activated and apoptotic and displayed more extensive mitochondrial metabolic dysfunction, especially cells in CD8+ T cell lineages, than those from convalescent COVID-19 patients or healthy controls, but slightly disturbed mitochondrial metabolic activity was observed in non–COVID-19 patients. Importantly, plasma IL-6 and C-reactive protein (CRP) levels positively correlated with mitochondrial mass and negatively correlated with fatty acid uptake in T cells from COVID-19 patients. Additionally, compared with those from healthy controls, in vitro–activated T cells from acutely infected COVID-19 patients showed signs of lower glycolysis, a reduced glycolytic capacity, and a decreased glycolytic reserve, accompanied by lower activation of mTOR signaling. Thus, newly identified defects in T cell mitochondrial metabolic functions and metabolic reprogramming upon activation might contribute to immune deficiency in COVID-19.