Descriptive norms (DNs) regarding physical activity (PA) (i.e., perceptions of how active others are) have been shown to positively relate to PA intentions (e.g., Rivis & Sheeran, 2003) and behaviors (e.g., Priebe & Spink, 2011). This relationship may be explained by the facilitation of self-efficacy (Bandura, 2004), which has been found to be a strong predictor of PA intentions (e.g., Hagger et al., 2001). According to Bandura, individuals can increase their self-efficacy by observing models who are perceived to be similar to them successfully perform that behavior (i.e., vicarious experiences). This notion allows one to posit that (1) DNs regarding significant others’ PA may operate as vicarious experiences that serve to increase one’s self-efficacy, and (2) this relationship may be moderated by one’s perceived similarity to the significant others. The purpose of the study was to test a hypothesized moderated mediation model: DNs positively predict PA intentions indirectly via self-efficacy (mediation), and the pathway from DNs to self-efficacy is stronger under conditions of higher perceived similarity (moderation). Data were collected via online surveys at two time points (N = 185, Mage = 25.9, SD = 10.3) and were analyzed via bootstrapping procedures (Preacher et al., 2007). The results indicated that PA intentions at time 1 predicted PA behaviors at time 2 (p = .003), and the indirect effect of DNs on PA intentions via self-efficacy was statistically significant (coefficient: .13, SE = .04, 95% CI: [.04, .21]). However, the overall moderated mediation was statistically nonsignificant (coefficient: .03, SE = .04, 95% CI: [-.05, .12]), suggesting nonsignificant moderation. The null results for moderation may be explained by ceiling effects of perceived similarity (M = 5.4, SD = 0.9 on a 7-point scale). The results supporting the mediation model imply that descriptive norm messages may be used to increase individuals’ PA self-efficacy. Such work may be particularly helpful for novice exercisers who lack previous PA experience and thus have low self-efficacy.