Background: Myxoid liposarcoma is a malignant tumor composed of uniform, partially round to oval cells with lipoblast, in the myxoid stroma with branching capillaries. Very rarely presents as a primary neoplasm of the subcutis or retroperitoneum. Myxoid liposarcoma in the retroperitoneum is the most common metastases. Case description: We reported a case of Mr M, male, 48 years old, came to the hospital. Private in the city of Medan on April 19 2021 with complaints of upper left abdominal pain, nausea (+), vomiting (-), fever. vital sign, general condition: compos mentis, temperature: 36 ℃, Pulse: 80 x/min. Breathing: 20x/min, Blood pressure: 120/80 mmHg. Previously, a gastric biopsy was performed on suspicion of gastric carcinoma and a histopathological examination was carried out with the results of a gastric adenoma. Subsequently, an exploratory laparotomy was performed with tumor debulking and the tissue was sent to the H. Adam Malik Hospital unit. On macroscopic examination, the tissue was split in a state of 16 cm x 12 cm x 7 cm, yellowish-gray in color, chewy consistency. The of cutting, a solid mass of white, yellow, and brown color appears, the consistency is chewy and brittle. With a tissue-weight of 800 grams then labeled O/1260/21. On microscopic examination of the tumor mass, it appears that the tumor mass is mostly arranged in solid sheets, consisting of round, relatively uniform cells, arranged back to back. Round and oval nuclei with increased N/C ratio, coarse chromatin, eosinophilic cytoplasm. Among them are lipoblast cells with spider web and small signet ring lipoblast. Atypical mitoses are relatively easy to find > 19 mitoses / 10 LPB. Proliferative and congested blood vessels, necrotic masses <50% are also seen. Immunohistochemical examination was carried out with S100 positive staining results on the nuclear membrane and cytoplasm of tumor cells with a proportion of more than 70% moderate to strong intensity. Conclusion: Based on macroscopic and microscopic examination this patient was diagnosed with High grade myxoid round cell liposarcoma grade III with ICD-O 8852/3.