Inguinal lymph node metastasis from adenocarcinoma arising at a colostomy site is extremely rare, and the significance of surgical resection for metastatic inguinal lymph nodes has not been established. An 82-year-old woman who had undergone abdominoperineal resection 27 years earlier was admitted to our hospital complaining of bleeding from a colostomy. Physical examination revealed that a tumor at the colostomy site directly invaded into the peristomal skin, and that a left inguinal lymph node was firm and swollen. Positron emission tomography/computed tomography scan demonstrated accumulation of F-18-fluorodeoxy glucose into both the colostomy tumor and the left swollen inguinal lymph node, while there was no evidence of metastasis to liver or lungs. She underwent open left hemicolectomy with wide local resection of the colostomy, and dissection of left inguinal lymph nodes. Histological diagnosis was a moderately differentiated adenocarcinoma that directly invaded into the surrounding skin and metastasized to the left inguinal lymph node. The patient has been followed up for >5 years without any sign of recurrence. In general, inguinal lymph node metastasis from colorectal cancers is regarded as a systemic disease with a poor prognosis, and so systemic chemotherapy and radiotherapy, but not surgical lymph node dissection, are recommended. Considering the lymphatic drainage route in the present case, inguinal lymph node metastasis does not represent a systemic disease but rather a sentinel nodal metastasis from adenocarcinoma at a colostomy site. Surgical dissection of metastatic inguinal lymph nodes should be considered to enable a favorable prognosis in the absence of distant metastasis to other organs.