Purpose Research on the longitudinal association of workplace social capital and mental health is limited. The purpose of the present study was to investigate the prospective association of social capital in the workplace with mental distress, measured by K6, adjusting for individuals factors as well as workplace-related factors among employees in Japan.

Methods The participants included 6387 men and 1825 women from 12 private companies in Japan. Questionnaires, inquiring about workplace social capital, K6, job strain and effort-reward imbalance were administered at the baseline survey between October 2010 and December 2011 (response rate=77.4%). At 1-year follow-up, social capital and K6 were assessed again (follow-up rate=79.5%), and a generalised linear model was used to estimate the association between changes in workplace social capital and change in K6.

Results After adjusting for baseline demographic characteristics and workplace-related factors (Job Content Questionnaire (JCQ), Effort-Reward Imbalance Questionnaire (ERIQ)), increased workplace social capital between waves was associated with improved psychological distress (β=−0.2327, p<0.0001). An inverse association was found in both men and women, all age groups, and among employees with high or low baseline mental health. The association was stronger among those who reported higher stress at baseline.

Conclusions Boosting workplace social capital may promote mental health in the workplace.