The pathophysiological role of Nox4 remains elusive and controversial. Even though Nox4 expression is increased in hypertension, diabetes and obesity, some studies demonstrate that Nox4 deficiency predisposes to obesity and insulin resistance. We showed that Nox1/4 inhibition is renoprotective in diabetes. Whether Nox4 influences vascular function and kidney status in obesity associated with mild diabetes is unclear. We hypothesized that Nox4-specific inhibition is protective against obesity-associated vascular and kidney damage. We studied db/m (control) and db/db mice (obese) mice for 16 weeks receiving 1) vehicle; 2) low dose Nox4 inhibitor, GKT137831 (GKT) (20 mg) and 3) high dose GKT (60 mg) for 16 weeks. Body weight increased in db/db (61.8g ± 0.95) versus controls (33.5g ± 0.72, p<0.05). Plasma glucose and albuminuria were slightly increased in db/db. GKT did not influence body weight but reduced epididymal fat mass in db/db (20% increase, p<0.05). Blood pressure was similar in all groups. Kidney weight (25%) and markers of renal fibrosis, such as, fibronectin (60%), pro-collagen I (PCI - 50%) and TGFβ (30%), were increased in db/db versus controls (p<0.05). High dose GKT decreased renal expression of fibronectin and PCI in db/db. Renal ERK1/2 activation was ameliorated by high dose GKT in db/db (vehicle, 65% vs GKT, 2.3%, p<0.05). Plasma 8-isoprostanes (db/db: 1263±96 vs db/m: 936±35), marker of systemic oxidative stress, and kidney hydrogen peroxide (db/db: 1 fold increase vs db/m) levels were increased in db/db, effects blocked by GKT. Endothelial dysfunction in db/db mice was not affected by GKT, but decreased NE-induced vascular contraction in db/db (pD2: db/db vs. db/db high dose, 6.4±0.1 vs. 5.6 ± 0.1, p<0.001). Reduced contraction by GKT was associated with decreased Rho kinase activity (db/db: 84% increase vs db/db+GKT 60 mg: 29% increase, p<0.05). Our findings suggest a role for Nox4 in obesity-associated kidney and vascular damage/dysfunction, important factors predisposing to cardiovascular disease in obesity. Mechanisms underlying Nox4 effects involve oxidative stress and Rho kinase.