Many G protein-coupled receptors form dimers in cells. However, underlying mechanisms are barely understood. We report here that intracellular factor XIIIA transglutaminase crosslinks agonist-induced AT1 receptor homodimers via glutamine315 in the carboxyl-terminal tail of the AT1 receptor. The crosslinked dimers displayed enhanced signaling and desensitization in vitro and in vivo. Inhibition of angiotensin II release or of factor XIIIA activity prevented formation of crosslinked AT1 receptor dimers. In agreement with this finding, factor XIIIA-deficient individuals lacked crosslinked AT1 dimers. Elevated levels of crosslinked AT1 dimers were present on monocytes of patients with the common atherogenic risk factor hypertension and correlated with an enhanced angiotensin II-dependent monocyte adhesion to endothelial cells. Elevated levels of crosslinked AT1 receptor dimers on monocytes could sustain the process of atherogenesis, because inhibition of angiotensin II generation or of intracellular factor XIIIA activity suppressed the appearance of crosslinked AT1 receptors and symptoms of atherosclerosis in ApoE-deficient mice.