Adiponectin is an important antiatherogenic adipocytokine that inhibits inflammation, insulin resistance, and oxide stress. Inflammation in the vascular adventitia is a crucial factor in the pathogenesis of atherosclerosis. Adventitial fibroblasts (AFs) can proliferate, divide into myofibroblasts, and migrate to the intima to become a new component of atherosclerotic plaque under inflammation and atherosclerosis. We investigated whether adiponectin might prevent AFs from proliferating, migrating, and transforming into myofibroblasts. Cultured AFs were stimulated with lipopolysaccharide (LPS) in the presence or absence of adiponectin. Methyl thiazolyl tetrazolium assay and migration and scratch-wound assays demonstrated that adiponectin reduced the AF proliferation and migration induced by LPS, respectively, whereas treatment with AdipoR1 small interfering (si) RNA (siAdipoR1), AMP-activated protein kinase (AMPK) siRNA (siAMPK), and an AMPK inhibitor reversed the effect. Immunocytochemistry and Western blot revealed that adiponectin reduced the transition of AFs to myofibroblasts, and treatment with siAdipoR1, siAMPK, and the AMPK inhibitor increased the transition. RT-PCR, Western blotting, and nitric oxide (NO) assay showed that adiponectin reduces induced NO synthase (iNOS) and nitrotyrosine expression and NO and ONOO^- production induced by LPS. Treatment with siAdipoR1, siAMPK, and the AMPK inhibitor significantly attenuated adiponectin-induced phosphorylation of AMPK and its downstream target acetyl-coenzyme A carboxylase and up-regulated iNOS mRNA and protein expression, which resulted in a marked increase of NO and ONOO^- production. In apolipoprotein E-deficient mice, immunohistochemistry of treated vascular adventitia showed that both iNOS expression and ONOO^- production could be reversed with an adenovirus-adiponectin vector. Taken together, these results suggest that adiponectin reduces LPS-induced NO production and nitrosative stress and prevents AFs from proliferating, transforming to myoflbroblasts, and migrating to the intima, thus worsening atherosclerosis, by inhibiting the AdipoR1-AMPK-iNOS pathway in AFs.