Background <p>Breast cancer survivors (BCS) are at an increased risk of diabetes and frequently experience weight gain post-diagnosis. This study examined the association between post-diagnostic weight changes and diabetes risk in BCS.</p> Methods <p>40,850 BCS without a history of diabetes were identified. Changes in obesity status (body mass index ≥ 25 kg/m²) and weight were assessed during general health examinations conducted before and after cancer diagnosis. The primary outcome was incident diabetes. Adjusted hazard ratios (aHRs) and confidence intervals (CIs) were calculated after adjusting for traditional diabetes risk factors, cancer treatments, and sociodemographic variables.</p> Results <p>During a mean follow-up of 4.6&#xa0;years, transitioning from non-obese to obese was associated with a 45% increased diabetes risk (aHR 1.45, 95% CI 1.16–1.82). Persistent obesity was associated with an 87% increased diabetes risk (aHR 1.87, 95% CI 1.64–2.15). The strength of the association decreased in individuals who reverted from obese to non-obese (aHR 1.42, 95% CI 1.18–1.71). Substantial weight loss (&gt; 10%) after diagnosis was associated with a 45% decreased risk of diabetes (aHR 0.55, 95% CI 0.43–0.70). In contrast, moderate weight gain (5–10%) was associated with an 18% increased risk (aHR 1.18, 95% CI 1.02–1.38).</p> Conclusions <p>Our findings suggest that transitioning to or maintaining an obese status post-diagnosis is associated with an increased diabetes risk, while substantial weight loss post-diagnosis is associated with a notable decrease in diabetes risk. Addressing obesity could significantly decrease the incidence of new-onset diabetes after breast cancer development.</p> Implications for Cancer Survivors <p>Breast cancer survivors are advised to maintain a healthy body weight after their diagnosis to help&#xa0;reduce the risk of diabetes.</p> Graphical abstract <p></p>