Abstract Background Portal venous gas has traditionally been considered an inevitable harbinger of death due to its association with bowel necrosis. Recently, an increasing number of cases of portal venous gas have been reported in patients with various clinical conditions and without bowel necrosis. We herein report the case of a patient in whom portal venous gas developed after transverse colon cancer surgery. Case presentation A 69-year-old man who had transverse colon cancer underwent insertion of a transanal ileus tube for decompression. Transverse colon resection was performed on the 11th day after the insertion of the transanal ileus tube. The patient had a high fever on the 6th day after the operation. Computed tomography showed portal venous gas over the entire area of the liver and pneumatosis intestinalis in the wall of the ascending colon. There were no signs of anastomotic leakage or bowel necrosis, so we decided to use conservative therapy with fasting and antibiotics. The portal venous gas disappeared on the 19th day after the operation. The patient was discharged in good condition on the 29th day after the operation. Conclusions Conservative treatment for portal venous gas is reasonable for patients without signs of anastomotic leakage or bowel necrosis. However, it is important to carefully observe patients with portal venous gas during conservative treatment because portal venous gas may be life-threatening.