A 69-year-old woman received radiation therapy of 50 Gy following surgery for left breast cancer. Eleven months later, chest computed tomography revealed infiltrative shadows with air bronchograms in both lower lung fields and a reversed halo sign in the right lower lobe. Transbronchial lung biopsy revealed polypoid granulation tissue in the terminal air spaces, which was consistent with organizing pneumonia (OP). Prednisolone therapy resolved the radiographic abnormalities. The reversed halo sign was believed to be specific to cryptogenic organizing pneumonia. We reported a case of OP with a reversed halo sign following postoperative irradiation for breast cancer.