The primary objective of this study was to evaluate the effect of GLPG3970 compared to placebo on the signs and symptoms of Ulcerative Colitis (UC) in participants with moderately to severely active UC.

Key Inclusion Criteria:

1. Documented diagnosis of UC of ≥3 months. The criteria for documentation of UC diagnosis based on endoscopy will be medical record documentation, and/or a colonoscopy report dated ≥3 months before screening, which shows features consistent with UC.

2. Treatment-experienced participants with moderately to severely active disease, who have either previously demonstrated inadequate clinical response, loss of response, or intolerance to at least 1 course of standard-of-care (SoC) therapy for UC (i.e. steroids [oral or parenteral, including but not limited to prednisone, prednisolone, budesonide], 5-aminosalicylate [5- ASA] derivatives [including but not limited to mesalamine, sulfasalazine], anti-metabolites [including but not limited to azathioprine, 6 mercaptopurine, methotrexate], anti-tumor necrosis factor [TNF] agents, anti-integrins, Janus kinase [JAK] inhibitors), as confirmed by the investigator.

3. Moderately to severely active UC as determined at screening by:

   1. Centrally-read endoscopic evidence of disease activity (MCS- endoscopy subscore [ES] ≥2 OR ulcerative colitis endoscopic index of severity [UCEIS] ≥4) with a minimum disease extent of 15 cm from anal verge; AND
   2. MCS stool frequency (SF) subscore ≥1; AND
   3. MCS rectal bleeding (RB) subscore ≥1.

4. Participants currently receiving the following SoC therapies for UC are eligible providing they have been on a stable dose for the designated period of time and are anticipated to be stable throughout the study:

   1. oral corticosteroids (prednisone ≤20 mg/day or equivalent or budesonide ≤3 mg/day) stable dose for at least 2 weeks prior to first investigational product (IP) dosing.
   2. oral 5-ASA compounds (mesalamine ≤4 grams [g]/day or sulfasalazine ≤4 g/day) stable dose for at least 4 weeks prior to first IP dosing.
   3. oral thiopurines (azathioprine ≤2.5 mg/kg/day and 6-mercaptopurine 1.5 mg/kilograms [kg]/day) stable dose for at least 12 weeks prior to first IP dosing, or methotrexate ≤20 mg/week, stable dose for at least 12 weeks prior to first IP dosing.

Key Exclusion Criteria:

1. Diagnosis of Crohn's disease, indeterminate colitis, ischemic colitis, fulminant colitis, or toxic megacolon.

2. Prior surgical intervention for UC (e.g. colectomy, partial colectomy, ileostomy or colostomy) or likely requirement for surgery for UC, during the study.

3. History or evidence of incompletely resected colonic mucosal dysplasia.

4. Exhibit acute severe UC per the following criteria:

   1. bloody diarrhea ≥6/day AND
   2. any of the following signs of systemic toxicity: Body temperature (oral or tympanic) ≥37.8 degrees celsius (°C) OR Resting pulse (after 5 min seated position) >90 beats per min OR hemoglobin <105 g/L, OR erythrocyte sedimentation rate >30 millimeters per hour (mm/h); OR C-reactive protein (CRP) >30 mg/L.

5. Screening stool sample positive for ova and/or parasites, Clostridium difficile toxin, Escherichia coli, Salmonella species (spp), Shigella spp, Campylobacter spp or Yersinia spp.

6. Participant testing positive at screening for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection as detected by real time polymerase chain reaction (RT-PCR), participants presenting any signs or symptoms as detected at baseline following careful physical examination (e.g. cough, fever, headaches, fatigue, dyspnea, myalgia, anosmia, dysgeusia, anorexia, sore throat, others) or reporting any signs and symptoms for the preceding 2 weeks, or participants who have been exposed to individuals with confirmed or suspected diagnosis of SARS-CoV-2 within 2 weeks prior to baseline. In addition, any other locally applicable standard diagnostic criteria may also apply to rule out SARS-CoV-2 infection.

Note: Other protocol-defined Inclusion/Exclusion criteria may apply.