Exposure to stress has a variety of consequences on human behavior and cognition. Although widely investigated, the impact of stress on working memory remains inhomogeneous. Individual differences in neuroendocrine responsiveness, for example, cortisol responses, may be factors that explain previous inconsistent results. This study assessed the role of cortisol responsiveness in the effects of psychosocial stress on working memory. To examine working memory processes, we analyzed both behavioral performances such as accuracy, response time, the inverse efficiency score, and event-related potentials (ERPs), including N1, P2, and P3. A total of 67 male college students completed a numerical 2-back task after being exposed to the Trier Social Stress Task (TSST) or a control task. The results showed shorter response time, better efficiency, and larger N1 and P2 amplitudes in the high-cortisol-responders compared to the low-cortisol-responders and the control group. This indicates a better working memory performance likely due to the enhancements in the orientation and mobilization of attention. Furthermore, a correlation analysis revealed a positive association between the cortisol change rate and the working memory performance and ERP data among the stressed individuals, suggesting that increased cortisol may facilitate working memory under acute psychosocial stress. These findings emphasize that the individual differences in cortisol responses may affect the impact of stress on working memory.