We present a case of ulcerative colitis (UC) in a patient during the first severe relapse with colonic dilatation and coexisting of giant renal tumor. Kidney tumor was constricting on colon and imitating toxic megacolon (TM). The patient with severe acute colitis (SAC) was admitted to the Department of Gastroenterology and Hepatology with inflammatory bowel disease (IBD) diagnosed in the previous month, based on clinical symptoms, endoscopy, and histopathology results. Computed tomography (CT) revealed a giant left kidney tumor without evidence of infiltration, a dilatation of the transverse colon up to 6 cm, and narrow light of the descending colon. There were no signs of intestinal obstruction or perforation. The control X-rays revealed maintaining megacolon, with dilatation of splenic flexure projection up to 6.5 to 7.5 cm. The patient was treated conservatively with no apparent improvement and finally operated on. Intraoperatively, a large tumor of the kidney (12 cm) constricting intestine was revealed. Left-sided nephrectomy and partial resection of the colon with the emergence of a colostomy was performed. The histopathology exam revealed renal mucinous tubular and spindle cell carcinoma (RMTSCC), a very rare malignant kidney tumor of low malignant potential and relative good prognosis. It was identified in the past 20 years. To date, approximately 100 such cases of cancer have been described.