There is little evidence that workplace social support can relieve workers' mental health problems. Therefore, we examined whether social support from coworkers and supervisors was associated with reduced serious psychological distress among employees. We used two-wave panel data from 13 Japanese companies. The baseline survey was conducted in 2011, and the follow-up survey 1 year later. From 9889 respondents, we selected 759 who had psychological distress at baseline, defined as ≥13 on the Kessler Psychological Distress Scale (K6). Increased coworker support was significantly associated with employees' reduced psychological distress (odds ratio, 3.51; 95% confidence interval, 2.17 to 5.68). The association between increased supervisor support and reduced psychological distress was nonsignificant (odds ratio, 1.32, 95% confidence interval, 0.85 to 2.04). Encouraging coworker support may contribute to the secondary prevention of mental health problems among employees.