Objectives: We investigated the potential of social support in an adherence intervention to foster physical activity (PA) among sedentary older adults. Methods: We compared 2, 9-week intervention programs aimed at increasing PA levels in older persons. One program offered PA alone, whereas the other combined a PA program with a social adherence intervention (PASAI). The interventions involved 39 sedentary older adults living in senior congregate housing in Israel, each randomly assigned to an intervention program. Information about PA levels, self-efficacy, and social support for exercise was collected through personal interviews before, after, and at a one-month follow-up. At follow-up, participants were asked to report whether they remained in contact with other group members. Results: PASAI program participants showed significantly higher levels of PA compared to PA program participants. Both self-efficacy and social support were associated with changes in PA. In the PASAI program, 67% of participants remained in contact with other group members; by contrast, PA participants did not maintain any contact. Conclusion: Self-efficacy perception and on-going social support encourage adherence to PA.