Revised idiopathic pulmonary fibrosis treatment guidelines were published in 2015, and nintedanib was conditionally recommended. Although diarrhea is reported to be a common major adverse event associated with nintedanib, there have been few reports on detailed endoscopic findings of nintedanib-associated enterocolitis. A 74-year-old woman was diagnosed with idiopathic pulmonary fibrosis 4 years ago in May. She was started on nintedanib (300 mg). Three months later, hepatic dysfunction was observed; therefore, the drug was temporarily discontinued and then resumed at a dose reduction of 200 mg. Five months later, the patient developed diarrhea, and the dose was reduced to 150 mg. However, no effect was noted; hence, colonoscopy was performed. Various inflammatory lesions, such as erythema and erosions, were observed continuously at the rectum, which resembled ulcerative colitis. No improvement was observed 2 months after follow-up colonoscopy, and nintedanib-related enterocolitis was suspected. The dose was further reduced to 100 mg. Since the endoscopic findings of nintedanib-associated enterocolitis are similar to those of ulcerative colitis, it is critical to consider patients with diarrhea who are taking nintedanib as having associated enterocolitis and attempt to reduce or discontinue the drug if diarrhea does not improve with antidiarrheal agents.