Gastrointestinal stromal tumor (GIST) is the most common non-epithelial tumor in the gastrointestinal tract. Although GIST occurs mainly in the gastrointestinal tract, it also occurs, rarely, in non-gastrointestinal tract and in this case, it is often named as extra-gastrointestinal stromal tumor (EGIST). We experienced a 68-year-old male patient who had been diagnosed preoperatively with accidentaloma of the left adrenal gland by computed tomography, and finally diagnosed as primary retroperitoneal malignant GIST, postoperatively. The operation was performed via anterior abdominal approach, and complete surgical resection was done for a 7 cm sized retroperitoneal tumor near the left adrenal gland. Primary retroperitoneal malignant GIST was the final pathologic diagnosis and the size of the tumor was 6.5 cm and the mitotic count was 7 per high-power field. Diffuse strong positive staining for c-kit protein, CD34 and negative staining for desmin were observed in a immunohistochemistry test. We report here the unusual case of primary retroperitoneal malignant GIST mimicking adrenal mass.