Abstract Background: Physical activity probably protects against the risk of breast cancer after menopause, but questions remain about how rapidly and for how long this protective effect exists. Methods: We analyzed data from 59,308 postmenopausal women (2,155 incident invasive breast cancers) followed between 1993 and 2005 (8.5 years postmenopause on average) through biennial questionnaires. Multivariable Cox models included time-varying exposure data, using levels of recreational physical activity self-reported in 1993, 1997, and 2002. Results: Women with recent (within the previous 4 years) recreational physical activity levels ≥12 metabolic equivalent task-hours (MET-h)/week had a lower risk of invasive breast cancer than women with lower levels [HR, 0.90; 95% confidence interval (CI), 0.82–0.99], with no apparent dose–response relation beyond 12 MET-h/week. Associations did not vary significantly across ER/PR subtypes. Risk reductions were of the same magnitude order regardless of weight change, body mass index, waist circumference, or less recent (5–9 years earlier) physical activity levels. Among women with levels of physical activity ≥12 MET-h/week 5 to 9 years earlier, those who became less active (<12 MET-h/week) had a significantly increased risk of breast cancer compared with those who did not (HR, 1.16; 95% CI, 1.01–1.35). And, compared with the least active women at both time points, they had no significantly decreased risk of breast cancer (HR, 1.06; 95% CI, 0.87–1.29). Conclusions: Our results suggest a decrease in risk associated with recent recreational physical activity even of modest levels. Impact: Starting or maintaining physical activity after menopause may be beneficial regarding breast cancer risk. Cancer Epidemiol Biomarkers Prev; 23(9); 1893–902. ©2014 AACR.