The present study examined the moderating (buffering) influence of physical fitness on the positive relation between job-related self-control demands and psychological strain. Data from 819 participants were obtained during a voluntary medical checkup. Physical fitness was assessed by a well-established measure of maximal oxygen uptake (expressed as milliliters of oxygen consumed per kilogram of body weight per minute); all other study variables were assessed by validated self-report measures. Hierarchical moderated regression analyses provided support for the prediction that physical fitness buffers the adverse impact of self-control demands on various indicators of psychological strain (burnout, ego depletion, need for recovery). For employees with low levels of physical fitness, the adverse impact of self-control demands on strain was much more pronounced than for those with high levels of physical fitness. The moderating effect of physical fitness suggests preventive interventions, which promote fitness and health, as a stress buffer especially among employees who have to meet high self-control demands at work.