Leiomyosarcoma (LMS) of sigmoid colon is extremely rare high grade neoplasm with poor prognosis. Gastrointestinal leiomyosarcoma are aggressive mesenchymal tumors. Here we present an unusual case of leiomyosarcoma of sigmoid colon with adhesions and perforation in small intestine. Case presentation: A 65 years old male patient referred to our institute with complaints of vomiting, pain and distention of abdomen .Clinical examination showed rigidity and guarding of abdomen with diminished bowel sounds. Clinically diagnosed as intestinal obstruction. Colonoscopy revealed growth at recto-sigmoid junction. Histopathology of biopsy reported with differential diagnosis. Malignant gastrointestinal stromal tumors (GIST) and Leiomyosarcoma. Immunohistochemistry confirmed the final diagnosis. SMA positive and CD117 negative. Hence diagnosed as leiomyosarcoma of rectosigmoid colon. Conclusion: Colonic leiomyosarcoma is rare and its occurrence in rectosigmoid colon is unusual. Leiomyosarcoma can be differentiated from GIST by IHC marker.