Nivolumab is an immune checkpoint inhibitor (ICI), which is used for treatment of advanced malignancies. Diarrhea is a typical adverse effect of ICIs. Several endoscopic features of ICI-related colitis have been reported. Herein, we reported a case of ICI-related colitis, with successful follow-up using ultrasonography (US). A 54-year-old woman, diagnosed with renal cell carcinoma, underwent radical nephrectomy. Multiple liver and bone metastases appeared 6 months after surgery. She was prescribed sunitinib, which was discontinued, because of severe diarrhea. She was prescribed nivolumab after 1 week. However, she was hospitalized 9 days later, due to fever and diarrhea. US showed diffuse wall thickening from the cecum to the rectum. The wall layer structures were preserved, and thickening was observed mainly in the submucosa, but not in the mucosa, without a decrease in the echo level. Subsequently, sigmoidoscopy was performed. Severe edema was observed in the mucosa, without any erosion and granularity. She was clinically diagnosed with ICI-related colitis, and steroid therapy was administered. One week after steroid therapy, the diarrhea improved, and the earlier findings showed marked improvement on US. After another week of reduced steroid dose, US revealed further improvement, and she was discharged. Keywords Immune checkpoint inhibitor. Nivolumab. Immune-related adverse effect. Colitis. Ultrasonography This article is part of the Topical Collection on Imaging