Dermatomyositis (DM) is a rare idiopathic inflammatory myopathy characterized by cutaneous and muscle manifestations. The association between DM and malignancy has been well recognized for many years. The clinical course of paraneoplastic DM may be affected by malignancies, although the cause and effect relationship between exacerbation of DM and cancer progression is uncertain. Herein, we report a 44-year-old woman who presented with progressive DM associated with rectal cancer. After curative resection of rectal cancer, DM symptoms resolved. Three months after surgery, blood test surveillance showed elevation of serum carcinoembryonic antigen levels, although the patient remained asymptomatic. One month later she had a DM flare-up, and multiple lung and liver metastases were found. She immediately underwent cancer chemotherapy with prednisolone therapy for DM. However, her condition deteriorated and she was unable to swallow. Percutaneous endoscopic gastrostomy was constructed, allowing alimentation and oral delivery, which made it possible to keep her on chemotherapy. She had remarkable response for unresectable metastases 8 weeks after the administration of chemotherapy. Seven months after onset of recurrence, her condition improved considerably and she had stable disease. Moreover, she can now eat food of soft consistency. Our case provides further support for the clinical importance of cancer chemotherapy for patients who have progressive DM and unresectable rectal cancer.