Aging is associated with a progressive loss of the CD28 costimulatory molecule in CD4(+) lymphocytes (CD28(null) T cells), which is accompanied by the acquisition of new biological and functional properties that give rise to an impaired immune response. The regulatory mechanisms that govern the appearance and function of this cell subset during aging and in several associated inflammatory disorders remain controversial. Here, we present the whole-genome DNA methylation and gene expression profiles of CD28(null) T cells and its CD28(+) counterpart. A comparative analysis revealed that 296 genes are differentially methylated between the two cell subsets. A total of 160 genes associated with cytotoxicity (e.g. GRZB, TYROBP, and RUNX3) and cytokine/chemokine signaling (e.g. CX3CR1, CD27, and IL-1R) are demethylated in CD28(null) T cells, while 136 de novo-methylated genes matched defects in the TCR signaling pathway (e.g. ITK, TXK, CD3G, and LCK). TCR-landscape analysis confirmed that CD28(null) T cells have an oligo/monoclonal expansion over the polyclonal background of CD28(+) T cells, but feature a V family repertoire specific to each individual. We reported that CD28(null) T cells show a preactivation state characterized by a higher level of expression of inflammasome-related genes that leads to the release of IL-1 when activated. Overall, our results demonstrate that CD28(null) T cells have a unique DNA methylation landscape, which is associated with differences in gene expression, contributing to the functionality of these cells. Understanding these epigenetic regulatory mechanisms could suggest novel therapeutic strategies to prevent the accumulation and activation of these cells during aging.