CD8 T cells, a subset of T cells identified by the surface glycoprotein CD8, particularly those expressing the co-stimulatory molecule CD226, play a crucial role in the immune response to malignancies. However, their role in chronic lymphocytic leukemia (CLL), an immunosuppressive disease, has not yet been explored. We studied 64 CLL patients and 25 age- and sex-matched healthy controls (HCs). We analyzed the proportion of CD226-expressing cells among different CD8 T cell subsets (including naïve, central memory, effector memory, and effectors) in CLL patients, stratified by Rai stage and immunoglobulin heavy-chain variable region gene (IgHV) mutation status. Additionally, we compared the effector functions of CD8CD226 cells and their CD226 counterparts. We also quantified cytokine and chemokine levels in the plasma of CLL and HCs. Furthermore, we reanalyzed the publicly available bulk RNA-seq on CD226 and CD226CD8 T cells. Finally, we evaluated the impact of elevated cytokines/chemokines on CD226 expression. Our results showed that CD226-expressing cells were significantly decreased within the effector memory and effector CD8 T cell subsets in CLL patients with advanced Rai stages and unmutated IgHV, a marker of poor prognosis. These cells displayed robust effector functions, including cytokine production, cytolytic activity, degranulation, proliferation, and migration capacity. In contrast, CD8CD226 T cells displayed an exhausted phenotype with reduced Runt-related transcription factor 2 (RUNX2) expression. Elevated levels of interleukin-6 (IL-6) and macrophage inflammatory protein-1 beta (MIP-1β) were inversely correlated with the frequency of CD8CD226 T cells and may contribute to the downregulation of CD226, possibly leading to T cell dysfunction in CLL. Our findings highlight the critical role of CD8CD226RUNX2 T cells in CLL and suggest that their reduction is associated with disease progression and poor clinical outcomes. This study also underscores the potential of targeting IL-6 and MIP-1β to preserve polyfunctional CD8CD226 T cells as a promising immunotherapy strategy.