Introduction: Preoperative image-based diagnosis is important for the treatment of rare cases of T1 lower rectal cancers with lateral pelvic lymph node (LLN) metastasis. We report a case of LLN metastasis in T1 lower rectal cancer diagnosed preoperatively via magnetic resonance imaging (MRI). Case presentation: A 65-year-old woman was admitted to our hospital because of abdominal pain. An endoscopic examination revealed a large laterally spreading tumor in the lower rectum, which was en bloc resected using endoscopic submucosal dissection. Pathological examination of the resected specimen showed deep invasion of the cancer cells into the submucosal layer and lymphovascular invasion. MRI revealed swollen perirectal lymph nodes (â¥5 mm) and a left LLN approximately 8 mm long. Laparoscopic abdominoperineal resection (Lap-APR) with left lateral pelvic lymph node dissection (LLND) was performed. Cancer cells were not seen in the resected material; however, 7 perirectal lymph nodes and 1 LLN of 47 lymph nodes contained metastatic cancer cells. Conclusion: We show that LLN metastasis in T1 lower rectal cancer can be preoperatively detected via MRI and successfully and safely treated via Lap-APR with left LLND.