Granulomatous mastitis (GM) is a benign inflammatory breast disease that often poses diagnostic challenges due to its similar clinical and radiographic features to breast cancer. We report the case of a 34-year-old female with concurrent GM and ductal carcinoma in situ (DCIS). Initially, breast cancer was suspected based on imaging; however, a needle biopsy confirmed GM. Corticosteroid treatment led to a reduction in tumor size, but subsequent imaging continued to suggest the presence of breast cancer. Surgical excision ultimately revealed the coexistence of GM and DCIS. It is essential to consider the possibility of concurrent breast cancer in cases of GM with discordant imaging and pathology findings.