Duodenal graft complications are not uncommon after pancreas transplant (PTx). Although direct visualization and biopsy of the duodenal graft are important for accurate diagnosis and management, endoscopic access is often limited in cases of enteric-drained PTx. Herein, we present a case of cytomegalovirus (CMV) graft duodenitis that was successfully diagnosed by transanal endoscopy using the double-balloon technique. The patient was a 54-year-old woman who underwent simultaneous pancreas and kidney transplant for type 1 diabetes mellitus and end-stage kidney disease. Enteric drainage was established by anastomosing the graft duodenum to her ileum. One month after the transplant, she developed fever and complained of lower abdominal pain. Graft duodenitis was suspected by laboratory test and imaging study results. Transanal double-balloon endoscopy was performed, and the biopsy specimen of the mucosa of the graft duodenum revealed CMV duodenitis without histopathologic findings of acute rejection. The postendoscopy course was uneventful. Treatment with ganciclovir was promptly initiated, and the CMV duodenitis was resolved with good function of the pancreas graft. In patients who undergo PTx with establishment of exocrine drainage by enteroanastomosis to the recipient ileum, transanal double-balloon endoscopy might be a feasible and safe technique for the surveillance of duodenal graft complications, including CMV duodenitis.