Necrotizing soft tissue infection (NSTI) is a rare but rapidly progressing soft-tissue infection. Few reports of NSTI caused by colon cancer have been published. We present a rare case of NSTI of the thigh associated the retroperitoneal spread of ascending colon cancer. A 64-year-old man had noticed right hip pain since 3 months before admission, he felt pain in the right thigh which was reddening, and he had difficulty in walking. He was referred to Yokosuka general hospital Uwamachi. Anterolateral aspect of his right thigh was reddening and swelling. The patient was diagnosed with a psoas abscess and a NSTI of the right thigh caused by penetration of ascending colon tumor. The patient underwent debridement of severely necrotized tissue in the right thigh, diverting ileostomy and subsequently a right hemicolectomy with reversal of the ileostomy were performed. He was discharged 70 days after the first surgery. Colon cancer can be a cause of retroperitoneal abscess accompanied by NSTI of the thigh. Two-stage surgery was an efficient option in this patient with NSTI of the thigh associated with locally advanced ascending colon cancer.