Mastitits caused by nontuberculous mycobacteria (NTM) is an unusual disease. It typically results from trauma or surgery, and rapid-growing NTM (RGM) is usually the causative pathogen. Mycobacterium conceptionense previously has been reported to cause soft tissue disease, but not mastitis. Herein, we report the case of a patient who contracted M. conceptionense mastitis after breast surgery and was successfully treated with nine months of a combination antibiotic regimen and no further surgical intervention.  (J Intern Med Taiwan 2019; 30: 408-413)