The pathogenesis of seborrheic dermatitis (SD) has not been clearly identified, and many factors are thought to play a role in its development. Recently, new studies have focused on increased oxidative stress (OS) in T cell-mediated skin diseases like psoriasis, contact dermatitis, and atopic dermatitis. However, there is no study investigating the status of OS in SD. In this study, we aimed to determine the status of OS in SD and the correlation of disease severity with OS. Fifty-four patients who were clinically and/or histopathologically diagnosed with SD were included in the study. Fifty-four healthy volunteers constituted the control group. Disease severity in patients with SD was scored according to the Seborrheic Dermatitis Area and Severity Index (SDASI). Serum total antioxidant status (TAS) and total oxidative status (TOS) were measured, and the oxidative stress index (OSI) was calculated in all patients and control subjects. The mean TAS values were significantly lower in the patient group than in the control group (p = 0.024). However, patients had significantly higher TOS and OSI values than the controls (p < 0.05). There was no correlation between SDASI and TAS, TOS, and OSI values. In this study, the association of oxidative stress and disease activity has first investigated in seborrheic dermatitis. It was found that OS was significantly higher in SD patients than in healthy subjects. In conclusion, our findings point to the possible role of the OS for the etiopathogenesis of SD.