OBJECTIVE: To investigate whether workplace social capital buffers the association between job stress and smoking status. METHODS: As part of the Harvard Cancer Prevention Project's Healthy Directions--Small Business Study, interviewer-administered questionnaires were completed by 1740 workers and 288 managers in 26 manufacturing firms (84% and 85% response). Social capital was assessed by multiple items measured at the individual level among workers and contextual level among managers. Job stress was operationalized by the demand-control model. Multilevel logistic regression was used to estimate associations between job stressors and smoking and test for effect modification by social capital measures. RESULTS: Workplace social capital (both summary measures) buffered associations between high job demands and smoking. One compositional item--worker trust in managers--buffered associations between job strain and smoking. CONCLUSION: Workplace social capital may modify the effects of psychosocial working conditions on health behaviors.