Background:Self-efficacy, action control, and social support are considered to influence changes in physical activity levels in older adults. This study examines the relationship among these variables and explores the putative mediating and moderating mechanisms that might account for activity changes.Methods:A longitudinal study with 54 older adults (≥ 50 years of age) was carried out in Costa Rica. In a moderated mediation analysis, action control was specified as a mediator between self-efficacy and physical activity, whereas social support was specified as a moderator between self-efficacy and action control. Baseline physical activity, age, and sex were specified as covariates.Results:Action control mediated between self-efficacy and physical activity. An interaction between social support and self-efficacy on action control pointed to a synergistic effect at the first stage of the mediating process.Conclusions:The effect of self-efficacy on physical activity was partly explained by action control, providing evidence of action control as a proximal mediator of physical activity. Moreover, the moderator role of social support was confirmed: high social support appeared to compensate for low levels of self-efficacy.