Leiomyosarcoma of the mesocolon is a very rare disease. Previously, there was no recommendation for chemotherapy or radiotherapy for leiomyosarcoma of the mesocolon, and only surgical resection reportedly offers a chance for cure. However, the reported cases of leiomyosarcoma of the mesocolon were not distinguished from those of gastrointestinal stromal tumor (GIST), which is the same mesenchymal tumor; thus, there is a possibility that they include cases that should be diagnosed as GIST. We report a case of leiomyosarcoma of the sigmoidal mesocolon.