Social support and barrier self-efficacy are important variables for adults who have followed a cardiac rehabilitation program, but few studies have examined their interacting effects. Therefore, the purpose of this study was to examine whether social support moderated the relationship between barrier self-efficacy and physical activity in adults post-cardiac rehabilitation. Participants (N = 57, Mage = 63, 85.5% males, 14.5% females) were adults who previously completed a cardiac rehabilitation program. They wore an accelerometer (Actigraph GT3X) for a 9-day period and valid 7-day moderate to vigorous physical activity data were retained for the analyses. Participants completed a questionnaire assessing barrier self-efficacy and social support. Separate moderation analyses were conducted to examine if social support from either family, peers or spouse moderated the relationship between barrier self-efficacy and physical activity. Peer social support was a significant moderator of the self-efficacy - physical activity relationship (B = -2.43, 95% CI = -2.82, -0.13, R2-change = 9%). At low levels of self-efficacy, participants with high social support from peers had greater physical activity levels than participants with low social support. However, when self-efficacy was high, physical activity levels were similar across the peer social support spectrum. Social support from family (B = -1.67, 95% CI= -2.46, 0.72, R2-change = 3%) and spouse (B = -0.96, 95% CI= -2.44, 0.31, R2-change = 3%) were not significant moderators. Cardiac rehabilitation graduates with low levels of barrier self-efficacy appear to be more physically active when they report greater social support from peers. Future post-cardiac rehabilitation interventions could utilize the power of peers to help increase physical activity levels of individuals with lower confidence and motivation to overcome physical activity-related barriers.