Proteasomes are essential molecular complexes that regulate intracellular protein homeostasis by selectively degrading ubiquitinated proteins. Genetic mutations in proteasome subunits lead to proteasome-associated autoinflammatory syndromes (PRAAS) characterized by autoinflammation, partial progressive lipodystrophy, and, in certain cases, immunodeficiency. However, the molecular mechanisms by which proteasome dysfunction results in these phenotypes remain unclear. Here, we established a mouse model carrying a mutation in β5i (encoded by Psmb8) along with T-cell-specific β5 (encoded by Psmb5) deficiency (KIKO mice). The KIKO mice presented severe loss of mature T cells in the spleen but not in the thymus, with reduced proteasome activity leading to the accumulation of ubiquitinated proteins. The CD4+ T cells of KIKO mice presented impaired proliferative activity with cell cycle arrest in the G0/G1 phase following TCR engagement. T cells from KIKO mice underwent rapid cell death through apoptosis, as treatment of T cells with the caspase inhibitor Z-VAD rescued cell viability. Moreover, proteasome dysfunction induced apoptosis in T cells without affecting either mitochondrial functions or ER stress responses. Thus, our data provide insight into the molecular mechanisms underlying not only immunodeficiency in PRAAS patients but also T-cell deficiency associated with other disorders.