To better understand mechanisms of physical activity (PA) behavior change in breast cancer survivors, we examined mediation of a successful PA behavior change intervention by social cognitive theory (SCT) constructs. Our exploratory study randomized 41 breast cancer survivors to receive the 3-month intervention (INT) or usual care (UC). We used the Freedman and Schatzkin approach to examine mediation of intervention effect on PA 3 months postintervention by changes in SCT constructs from baseline to immediately postintervention. Compared with UC, the INT group reported lower barriers interference (mean difference = -7.8, 95% CI [-15.1, -0.4], d = -0.67, p = .04) and greater PA enjoyment (mean difference = 0.7, 95% CI [0, 1.5], d = 0.61, p = .06). Barriers interference mediated 39% (p = .004) of the intervention effect on PA 3 months postintervention. PA enjoyment was not a significant mediator. Reducing barriers to PA partially explained our intervention effect.