This is a multicenter, Phase 2, double-blind, placebo-controlled study in subjects with active Ankylosing Spondylitis (AS). A total of approximately 100 subjects will be randomized to one of 2 treatment arms in a 1:1 ratio: oral filgotinib tablets q.d. or matching placebo q.d. Treatment duration will be 12 weeks. Each subject will stay in the study for a maximum of 20 weeks (from Screening visit to Follow-up visit).

Key Inclusion Criteria:

• Male or female subjects who are ≥18 years of age on the day of signing informed consent.
• Diagnosis of moderate to severe ankylosing spondylitis with documented evidence of fulfilling the Modified New York (NY) criteria
• Have active ankylosing spondylitis with a BASDAI ≥4 (numeric rating scale [NRS] 0-10) and spinal pain ≥4 (0-10 NRS) (based on BASDAI question 2, see protocol) at screening and baseline.
• Have had a documented inadequate response to NSAIDs including cyclooxygenase-2 (COX-2) inhibitors.
• If using cDMARD therapy, stable dose for at lead 4 weeks prior to Baseline.
• If using non-drug therapies (including physical therapies), these should be kept stable during screening.
• Male and female subjects of childbearing potential who engage in heterosexual intercourse must agree to use highly effective methods of contraception as described in the protocol.

Key Exclusion Criteria:

• Use of JAK inhibitors, investigational or approved, at any time, including filgotinib;
• Prior use of more than one TNF inhibitor, at any time.
• Use of oral steroids at a dose >10 mg/day of prednisone or prednisone equivalent or at a dose that hasn't been stable for at least 4 weeks prior to baseline;
• Any therapy by intra-articular injections (e.g. corticosteroid, hyaluronate) within 4 weeks prior to screening;
• Use of more than 1 NSAID or COX-2 inhibitor.
• Contraindication to MRI.
• History of known or suspected complete ankylosis of the spine.
• Presence of very poor functional status or unable to perform self-care.
• Have undergone surgical treatment for ankylosing spondylitis within the last 12 weeks prior to screening.
• Administration of a live or attenuated vaccine within 12 weeks prior to baseline.