• Approximately 60 patients suffering from moderate to severe ulcerative colitis will be evaluated for improvement of disease activity (efficacy) when taking GLPG1205 or matching placebo once daily for 12 weeks in addition to their stable background treatment.
• During the course of the study, patients will also be examined for any side effects that may occur (safety and tolerability), and the amount of GLPG1205 present in the blood (Pharmacokinetics) as well as the effects of GLPG1205 on disease- and mechanism of action-related parameters (Pharmacodynamics) in blood, stool and colonic biopsies will be determined.

Inclusion Criteria:

• Male or female subjects between 18 and 75 years
• Documented history of UC
• Presence of active UC for a minimum period of 14 days prior to screening and spread beyond the rectum, Mayo score ≥ 6 with rectal bleeding score ≥ 1 and endoscopy score ≥ 2
• Absence of infectious colitis
• Tumor necrosis factor alpha (TNFα) inhibitor-naive subjects should have failed at least 1 prior conventional therapy
• Continuation of concurrent treatment with oral steroids (≤30 mg prednisolone eq/day), immunosuppressants and 5-aminosalicylates at stable dose is allowed
• Female subjects must have a negative blood pregnancy test, unless they are surgically sterile, had a hysterectomy, or have been postmenopausal for at least 1 year
• Subjects will have to use highly effective contraceptive methods

Exclusion Criteria:

• History of sensitivity to any component of the study drug, or a history of drug or other allergy
• Any concurrent illness, condition, disability, or clinically significant abnormality that, in the investigator's opinion, represents a safety risk for the subject's participation, may affect the interpretation of data, or may prevent the subject from safely completing the assessments
• History of significant psychological, neurologic, hepatic, renal, endocrine, cardiovascular, GI (other than UC), pulmonary, or metabolic disease
• History of active infections requiring intravenous antibiotics within the past 4 weeks prior to screening.
• History of malignancy within the past 5 years; presence or history of intestinal malignancy
• History of bowel surgery within 6 months prior to screening; history of colon resection with < 30 cm of the colon remaining
• Suspicion of Crohn's disease, indeterminate colitis, microscopic colitis, ischemic colitis, diverticular disease-associated colitis, or radiation-induced colitis
• Subject who has received non-permitted UC therapies within specified timeframes, depending on the medication, as stated in the protocol
• Subject who is pregnant or lactating