Colorectal cancer (CRC) is one of the leading causes of death in Taiwan. Histopathologically, most CRC subtypes are classified as adenocarcinomas, and primary signet-ring cell carcinoma (PSRCC), which accounts for <1% of all cases of CRC is rare. Moreover, endoscopic detection of PSRCC is challenging. Colonic PSRCC, which is usually diagnosed at an advanced stage, is highly aggressive and shows poor prognosis. In this case report, we describe a 48-year-old man who presented with progressive left lower quadrant pain and was diagnosed with PSRCC at the rectosigmoid junction.