Abstract CYLD was originally identified as a tumor suppressor gene mutated in familial cylindromatosis, an autosomal dominant predisposition to multiple benign neoplasms of the skin known as cylindromas. The CYLD protein is a deubiquitinating enzyme that acts as a negative regulator of NF-κB and JNK signaling through its interaction with NEMO and TNFR-associated factor 2. We have previously described a novel mouse strain that expresses solely and excessively a naturally occurring splice variant of CYLD (CYLDex7/8). In this study, we demonstrate that CYLD plays a critical role in Treg development and function. T cells of CYLDex7/8 mice had a hyperactive phenotype manifested by increased production of inflammatory cytokines and constitutive activation of the NF-κB pathway. Furthermore, the amount of Foxp3+ regulatory T cells in these mice was markedly enhanced in thymus and peripheral organs. Importantly, these regulatory T cells displayed decreased expression levels of CD25 and CTLA-4 associated with impaired suppressive capacity. Hence, our data emphasize an essential role of CYLD in maintaining T cell homeostasis as well as normal T regulatory cell function, thereby controlling abnormal T cell responses.