Bowel ultrasound is a noninvasive alternative to endoscopy for assessing the disease activity of ulcerative colitis; however, it is unclear whether bowel ultrasound can predict subsequent relapse from remission. A retrospective cohort study enrolled patients with ulcerative colitis who underwent bowel ultrasound between July 2018 and July 2021 during clinical remission (patient-reported outcome-2 ≤1 and no rectal bleeding) for at least 3 months and were followed up for 1 year. Ultrasonographic findings (bowel wall thickness, bowel wall flow, bowel wall stratification, and enlarged lymph nodes), Milan ultrasound criteria, Mayo endoscopic subscore, C-reactive protein, and fecal calprotectin levels and their association with subsequent clinical relapse were assessed. Relapse was defined as rectal bleeding score ≥1, stool frequency score ≥2, or treatment intensification for symptoms. 31% of the patients (18/58) relapsed within 1 year. No single ultrasonographic finding predicted relapse, whereas Milan ultrasound criteria >6.2 (p = 0.019), Mayo endoscopic subscore ≥1 (p = 0.013), and fecal calprotectin ≥250 μg/g (p = 0.040) were associated with a shorter time to relapse in the log-rank test. Milan ultrasound criteria >6.2 (hazard ratio 3.22; 95% confidence interval 1.14-9.08, p = 0.027) and Mayo endoscopic subscore ≥1 (hazard ratio 8.70; 95% confidence interval 1.11-68.1, p = 0.039) showed a higher risk of relapse according to a Cox proportional hazards model. Bowel ultrasound can predict subsequent clinical relapse from remission in patients with ulcerative colitis using the Milan ultrasound criteria.