The stress hormone cortisol is important for the regulation of social motivational processes. High cortisol levels have been associated with social fear and avoidance, which play an important role in social anxiety disorder (SAD), as does hypervigilant processing of social threat. However, causal effects of cortisol on threat processing in SAD remain unclear. In an event-related potential (ERP) study we investigated the effects of cortisol on task-irrelevant (implicit) processing of social threat in SAD, exploring the temporal dynamics as well as the role of symptom severity and stimulus awareness. Angry face processing was measured in participants with clinical SAD after double-blind, within-subject oral administration of cortisol (50 mg) and placebo, using a masked and an unmasked emotional Stroop task. Both tasks showed significantly increased P2 midline ERP amplitudes for angry compared to neutral and happy faces in the placebo condition, reflecting an early attentional bias for social threat in SAD. Furthermore, cortisol administration significantly decreased P2 amplitudes for masked angry faces. This effect correlated with social anxiety, showing stronger decreases in patients with higher levels of social anxiety. These results indicate a highly specific effect of cortisol on early motivated attention to social threat and, together with previous findings, highlight the importance of motivational context (stimulus- or task-relevance) as well as symptom severity.