INTRODUCTION: Immune checkpoint inhibitors (ICIs), used in cancer immunotherapy, enhance the immune system's ability to attack cancer cells. However, this activation can lead to severe immune-associated adverse events due to overactivation. In autoimmune diseases, the immune system mistakenly targets the body's tissues, producing autoantibodies that cause inflammation and tissue damage. Despite the increasing use of ICIs, limited information exists on their effects and potential harms in patients with active autoimmune diseases, making it challenging to predict outcomes and manage risks for these patients. CASE PRESENTATION: We report a case of a patient with breast cancer presenting with a rash and muscle weakness. The simultaneous onset of these symptoms, along with the rapid growth of the breast tumor, led to a diagnosis of tumor-associated dermatomyositis (DM). The patient presented with locally advanced triple-negative breast cancer and received preoperative chemotherapy, including ICIs. CONCLUSION: The administration of preoperative chemotherapy, including ICIs, to a patient with breast cancer and tumor-associated DM was found to be a safe and effective treatment approach. There is a need to better understand the interplay between ICIs and autoimmune diseases and to develop safe and effective treatment strategies for this unique patient population.