We report an unusual case of uterine corpus metastasis from rectal adenocarcinoma detected using 18F-FDG PET/CT. A 47-year-old woman was diagnosed with rectal adenocarcinoma and treated with surgery and radiochemotherapy 3 years ago. During follow-up, she presented with lower abdomen pain and elevated tumor markers, but ultrasonography findings were unremarkable. We performed an 18F-FDG PET/CT scan, which showed intense 18F-FDG avidity in the myometrium of the uterine corpus. Uterine corpus metastasis was suspected, and complementary MRI and diagnostic curettage histopathology confirmed uterine metastasis from rectal adenocarcinoma.