LAB (linker for activation of B cells), also known as NTAL (non-T cell activation linker), is a LAT (linker for activation of T cells)-like adaptor protein that is expressed in B, NK, and mast cells. Its role in lymphocytes has not been clearly demonstrated. Here, we showed that aged LAB-deficient (Lat2-/-) mice developed an autoimmune syndrome. Lat2-/- T cells were hyperactivated and produced more cytokines than Lat2+/+ T cells. Even though LAB was absent in naive T cells, LAB could be detected in activated Lat2+/+ T cells. LAT-mediated signaling events were enhanced in Lat2-/- T cells; however, they were suppressed in T cells that overexpressed LAB. Mice with the Lat2 gene conditionally deleted from T cells also developed the autoimmune syndrome like Lat2-/- mice. Together, these data demonstrated an important role of LAB in limiting autoimmune response and exposed a mechanism regulating T cell activation.