Fracture and osteoporosis are known side effects of aromatase inhibitors (AIs) for postmenopausal hormone receptor positive (HR+) breast cancer (BC) patients. How modifiable lifestyle factors impact fracture risk in these patients is relatively unknown. We conducted a prospective cohort study to examine the association of lifestyle factors, focusing on physical activity, with risk of incident major osteoporotic fracture and osteoporosis in 2152 HR+ BC patients diagnosed from 2006 to 2013 at Kaiser Permanente Northern California and who received AIs. Patients self-reported lifestyle factors at study entry and at 6-month follow-up. Fracture and osteoporosis outcomes were prospectively ascertained by physician-adjudication and bone mineral density (BMD) values, respectively. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated from multivariable proportional hazards regression. Models were adjusted for age, menopausal status, race/ethnicity, body mass index (BMI), AJCC stage, breast cancer treatment, prior osteoporosis, and prior major fracture. Over a median 6.1 years of follow-up after AI initiation, 165 women experienced an incident osteoporotic fracture and 243 women had osteoporosis. No associations were found between overall moderate-vigorous physical activity and fracture risk, although < 150 min/week of aerobic exercise in the 6 months after BC diagnosis was associated with increased fracture risk (HR=2.42; 95% CI: 1.34, 4.37) compared with ≥ 150 min/week (meeting physical activity guidelines). Risk was also higher for never or infrequently engaging in aerobic exercise (HR=1.90; 95% CI: 1.05, 3.44). None or infrequent overall moderate-vigorous physical activity in the 6 months before BC diagnosis was associated with increased risk of osteoporosis (HR=1.94; 95% CI: 1.11; 3.37). Moderate-vigorous physical activity during the immediate period after BC diagnosis, particularly aerobic exercise, was associated with lower risk of major osteoporotic fractures in women on AI therapy. Findings may inform fracture prevention in women on AI therapy through non-pharmacologic lifestyle-based strategies.