Integrin-mediated adhesion plays a central role in T cell trafficking and activation. Genetic inactivation of the guanine nucleotide-binding (G) protein alpha-subunits Galpha(12) and Galpha(13) resulted in an increased activity of integrin leukocyte-function-antigen-1 in murine CD4(+) T cells. The interaction with allogeneic dendritic cells was enhanced, leading to an abnormal proliferative response in vitro. In vivo, T cell-specific inactivation of Galpha(12) and Galpha(13) caused lymphadenopathy due to increased lymph node entry and enhanced T cell proliferation, and the susceptibility toward T cell-mediated diseases was enhanced. Mechanistically, we show that in the absence of Galpha(12) and Galpha(13) the activity of the small GTPases Rac1 and Rap1 was increased, whereas signaling of the small GTPase RhoA was strongly reduced. Our data indicate that locally produced mediators signal through Galpha(12)- and Galpha(13)-coupled receptors to negatively regulate cell polarization and adhesiveness, thereby fine-tuning T cell trafficking, proliferation, and susceptibility toward T cell-mediated diseases.