Rationale: In rodent models of depression, oxidative stress-induced reductions in nitric oxide (NO) bioavailability contribute to impaired endothelium-dependent dilation. Endothelial dysfunction is evident in Major Depressive Disorder (MDD); however, the molecular mediators remain undefined. Objective: We sought to translate preclinical findings to humans by testing the role of oxidative stress in mediating microvascular endothelial dysfunction, including potential modulatory influences of sex, in MDD. Methods and Results: Twenty-four treatment-naïve, otherwise healthy, young adults with MDD (14 women; 18-23 yrs) and twenty healthy adults (HC; 10 women; 19-30 yrs) participated. Red blood cell flux (laser Doppler flowmetry) was measured during graded intradermal microdialysis perfusion of the endothelium-dependent agonist acetylcholine, alone and in combination with an NO synthase inhibitor (L-NAME), a superoxide scavenger (Tempol), and an NADPH oxidase inhibitor (apocynin), as well as during perfusion of the endothelium-independent agonist sodium nitroprusside. Tissue oxidative stress markers (e.g., nitrotyrosine abundance, superoxide production) were also quantified. Endothelium-dependent dilation was blunted in MDD and mediated by reductions in NO-dependent dilation. Endothelium-independent dilation was likewise attenuated in MDD. In MDD, there were no sex differences in either NO-mediated endothelium-dependent dilation or endothelium-independent dilation. Acute scavenging of superoxide or inhibition of NADPH oxidase improved NO-dependent dilation in MDD. Expression and activity of oxidative stress markers were increased in MDD. In a subset of adults with MDD treated with a selective serotonin reuptake inhibitor for their depressive symptoms and in remission (n=8; 7 women; 19-37 yrs), NO-mediated endothelium-dependent dilation was preserved, but endothelium-independent dilation was impaired, compared to HC.