<h3>Purpose/Objective(s)</h3> Spontaneous rib fracture (SRF) is a common late complication in treated breast cancer patients. This study evaluated the incidence and risk factors of ipsilateral SRF after radiotherapy (RT) in breast cancer patients. In addition, we identified dosimetric parameters that were significantly associated with ipsilateral SRF. <h3>Materials/Methods</h3> We retrospectively reviewed 2,204 patients with breast cancer who underwent RT between 2014 and 2016, and were followed up with bone scans. We evaluated clinical risk factors for ipsilateral SRF. Dose-volume histogram analysis was also performed for patients (n = 538) whose dosimetric data were available. All ipsilateral ribs were manually delineated, and dosimetric parameters of the ribs were converted into the equivalent dose in 2 Gy fractions (EQD2). <h3>Results</h3> Ipsilateral SRF occurred in 14.5% of patients 3 years after RT. The median time to develop ipsilateral SRF was 15 months. In multivariate analysis of all patients, a hypofractionated dose scheme and abnormally low bone density were significant clinical risk factors for ipsilateral SRF. In dosimetric analysis, near-maximum rib dose (D2cc) best predicted ipsilateral SRF. The cutoff value of D2cc was EQD2 52 Gy, as determined by receiver operating characteristic analysis. In multivariate analysis including dosimetric variables, D2cc EQD2 ≥ 52 Gy was the only significant risk factor for ipsilateral SRF. <h3>Conclusion</h3> Our data demonstrated that near-maximum rib dose was the best dosimetric parameter to predict ipsilateral SRF in RT-treated breast cancer patients. Additionally, we suggest that the dose for the rib be constrained to D2cc EQD2 < 52 Gy to minimize the risk of ipsilateral SRF.