Objective Social support is theoretically expected to be positively associated with posttraumatic growth (PTG) and subjective well‐being, and stress is expected to be positively associated with PTG and negatively associated with subjective well‐being among breast cancer (BC) survivors. However, empirical evidence is mixed, predominantly cross‐sectional, and few studies have examined the unique effects of these predictors on positive changes in psychological experiences post cancer diagnosis and systemic treatment. This study examined both general and BC‐specific social support and stress as predictors of change in PTG and subjective well‐being among BC survivors. Methods Women (N = 173, Mage = 55.40, SD = 10.99) who had recently finished treatment completed demographic and treatment measures at baseline (T1); general and cancer‐specific social support and stress, PTG and subjective well‐being at 3 months (T2); and PTG and subjective well‐being again at 6 months (T3). Longitudinal predictors of change in PTG and subjective well‐being were examined using hierarchical multiple regression. Results The BC‐specific social support (β = .12) and stress (cancer worry; β = .10) predicted increasing levels of PTG. Improvements in subjective well‐being were predicted by higher levels of general social support (β = .21) and lower levels of general stress (β = −.59). Conclusions There are distinct predictors of change in PTG and subjective well‐being among BC survivors, supporting the distinction between the trauma‐specific process of PTG and well‐being. Copyright © 2013 John Wiley & Sons, Ltd.