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Background: Epidemiologic studies have reported associations between weight fluctuations and postmenopausal breast cancer risk; however, the biological markers involved in this association are unknown. This study aimed to explore the associations between breast cancer–related biomarkers and weight regain following exercise-induced weight loss. Methods: From the 400 participants included in the Breast Cancer and Exercise Trial in Alberta, a total of 214 lost weight during the intervention and had follow-up blood samples, body composition, and covariate measurements. Outcomes were measured at baseline, 12 months (end of the study), and 24 months (follow-up). Results: During follow-up, weight regain was 1.80 kg [95% confidence interval (CI): –0.40–3.90], and was significantly associated with increases in estradiol [treatment effect ratio (TER) = 1.03; 95% CI, 1.01–1.04], estrone (TER = 1.02; 95% CI, 1.01–1.03), free estradiol (TER = 1.04; 95% CI, 1.02–1.05), the homeostatic model assessment for insulin resistance (TER = 1.03; 95% CI, 1.02–1.05), and insulin (TER = 1.03; 95% CI, 1.01–1.04), and decreases in sex hormone–binding globulin (SHBG; TER = 0.98; 95% CI, 0.97–0.99) levels. Nonstatistically significant associations were found for glucose and C-reactive protein. Furthermore, a statistically significant linear trend of increasing levels for all biomarkers, and decreasing SHBG, across weight regain categories was found. Conclusions: These results suggest that weight regain following exercise-induced weight loss is associated with breast cancer–related biomarker changes in postmenopausal women. Impact: These findings provide evidence to support the importance of developing effective strategies to prevent weight regain and, consequently, decrease postmenopausal breast cancer risk via changes in adiposity-related biomarkers.