Abstract The antigen-specific interaction of a T cell with an antigen-presenting cell (APC) results in the formation of an immunologic synapse (IS) between the membranes of the 2 cells. β2 integrins on the T cell, namely, leukocyte function-associated antigen 1 (LFA-1) and its counter ligand, namely, immunoglobulin-like cell adhesion molecule 1 (ICAM-1) on the APC, critically stabilize this intercellular interaction. The small GTPase Rap1 controls T-cell adhesion through modulating the affinity and/or spatial organization of LFA-1; however, the upstream regulatory components triggered by the T-cell receptor (TCR) have not been resolved. In the present study, we identified a previously unknown function of a protein kinase C-θ (PKC-θ)/RapGEF2 complex in LFA-1 avidity regulation in T lymphocytes. After T-cell activation, the direct phosphorylation of RapGEF2 at Ser960 by PKC-θ regulates Rap1 activation as well as LFA-1 adhesiveness to ICAM-1. In OT-II TCR-transgenic CD4+ T cells, clustering of LFA-1 after antigen activation was impaired in the absence of PKC-θ. These data define that, among other pathways acting on LFA-1 regulation, PKC-θ and its effector RapGEF2 are critical factors in TCR signaling to Rap1. Taken together, PKC-θ sets the threshold for T-cell activation by positively regulating both the cytokine responses and the adhesive capacities of T lymphocytes.