Purpose In this study, changes in motivational regulations in women following treatment for breast cancer were described. Changes in motivational regulations as predictors of subsequent change in light and moderate‐to‐vigorous physical activity (PA) and affect were also examined. Methods Women [n = 150; Mage = 54.41 (SD = 10.87) years] completed self‐report questionnaires and wore an accelerometer for 7 days at Time 1 [M = 3.94 (SD = 3.08) months following primary treatment], as well as 3 (Time 2) and 6 (Time 3) months later. Data were analyzed using repeated‐measures analysis of variance and path analysis using residual change scores. Results Identified regulation and self‐determined motivation (i.e., combined intrinsic motivation and identified regulation) scores decreased over time (p < 0.05). In the path model [χ2(4) = 5.66, p = 0.22, root mean square error of approximation = 0.05 (90% CI: 0.0; 0.15), comparative fit index = 0.99, standardized root mean square of the residuals = 0.03], ΔTime1–2 in external regulation was associated with ΔTime2–3 in positive affect (β = −0.16), ΔTime1–2 in introjected (β = 0.25) and amotivation (β = 0.19) were related to ΔTime2–3 in negative affect, and ΔTime1–2 in self‐determined motivation was related to ΔTime2–3 in positive affect (β = 0.40) and moderate‐to‐vigorous PA (β = 0.21). Conclusions Changes in motivational regulations were related to changes in PA and affect in the aftermath of breast cancer. Given the benefits of self‐determined motivation, additional research is needed to develop and test interventions aimed at enhancing this type of motivation. Copyright © 2013 John Wiley & Sons, Ltd.