Canine blastomycosis is a common systemic fungal infection within the Ohio and Mississippi River valleys and typically presents as pneumonia, lymphadenitis, or endophthalmitis. This report describes three cases in which mammary tissue samples were submitted to the Department of Pathobiology, University of Tennessee, College of Veterinary Medicine with clinical suspicion of neoplasia or postpartum bacterial mastitis. Pyogranulomatous to granulomatous mastitis and dermatitis with intralesional yeast consistent with Blastomyces dermatitidis were diagnosed. Two of the three dogs also had lymph node and pulmonary involvement. Mycotic mastitis due to Blastomyces dermatitidis is rarely reported and blastomycosis should be considered in the differential diagnosis of dogs with mammary lesions from endemic areas.