Granulomatous mastitis (GM) is a chronic inflammatory disease of the breast that usually occurs in women of reproductive age. However, GM during pregnancy is unusual and only one case of GM in the accessory breast has been reported so far. Here, we report an extremely rare case of GM in the accessory axillary breast of a pregnant woman. A 24-year-old pregnant woman had persistent pain and swelling in the right axilla that did not improve with antibiotic administration. Despite incision and drainage for subcutaneous abscess, the incised skin gradually became ulcerated, exposing the subcutaneous granulomatous tissue. Corynebacterium species were isolated in the bacterial culture of drained pus. Lower back pain, pain in several joints, and erythema nodosum on the lower legs appeared later. Based on the result of bacterial culture and the above disease course, the patient was clinically suspected of having GM. The axillary mass was surgically removed after childbirth, and the excised mass was histopathologically confirmed as GM. Treatment for GM should be considered individually and carefully in accordance with the patients' condition. Unnecessary surgery should be avoided. However, early addition of surgical interventions may yield good outcomes, especially for pregnant women because of limited treatment options.