BACKGROUND/AIMS: Filgotinib (FIL), a Janus kinase inhibitor, shows clinical efficacy in moderate to severe ulcerative colitis (UC), but no prospective studies have examined endoscopic and histopathological outcomes. This study aimed to evaluate the therapeutic efficacy of FIL in moderate to severe UC using the Partial Mayo Score (PMS), Ulcerative Colitis Endoscopic Index of Severity (UCEIS), and Geboes Histopathology Score (GHS). METHODS: Twenty-two patients with clinically moderate to severe refractory UC were enrolled. Remission was defined as PMS 0, UCEIS 0, and GHS < 2.0 (sigmoid and rectum). Achievement rates were prospectively evaluated at 12, 24, and 52 weeks after FIL initiation compared to baseline. RESULTS: Among the 22 patients, comprising Biologic-Naïve (BN,  = 12) and Biologic-Experienced (BE,  = 10) cohorts, achievement rates were highest for PMS 0, followed by UCEIS 0, and lowest for GHS < 2.0. Partial Mayo Score 0 achievement for BN/BE was 75% ( = .001)/50% ( = .031) at 12 weeks, 75% ( = .003)/70% ( = .016) at 24 weeks, and 75% ( = .002)/70% ( = .016) at 52 weeks. Ulcerative Colitis Endoscopic Index of Severity 0 achievement for BN/BE was 58.3% ( = .008)/20% ( = .016) at 12 weeks, 41.6% ( = .019)/40% ( = .016) at 24 weeks, and 50% ( = .002)/50% ( = .016) at 52 weeks. Geboes Histopathology Score < 2.0 (sigmoid) achievement for BN/BE was 25%/0% at 12 weeks, 33.3%/10% at 24 weeks, and 25%/10% at 52 weeks. Geboes Histopathology Score < 2.0 (rectum) achievement for BN/BE was 50%/0% at 12 weeks, 41.6%/20% at 24 weeks, and 33.3%/40% at 52 weeks. CONCLUSIONS: Filgotinib appears to be an effective treatment for UC, demonstrating potential for achieving not only clinical remission but also endoscopic and histopathological remission.