This is a multicenter, Phase 2, double-blind, placebo-controlled study in subjects with moderately to severely active Psoriatic Arthritis (PsA) who have an inadequate response or are intolerant to conventional disease-modifying therapy. A total of approximately 124 subjects will be randomized to one of 2 treatment arms in a 1:1 ratio: oral filgotinib tablets q.d. or matching placebo tablets q.d. The Screening visit will occur within 28 days before study drug administration. At Day 1 (Baseline), eligible subjects will be randomized to treatment for a duration of 16 weeks. The study is concluded with a Follow-up period lasting until 4 weeks after the last dose. Consequently, each subject will stay in the study for a maximum of 24 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 psoriatic arthritis meeting Classification Criteria for Psoriatic Arthritis (CASPAR)
• Have active psoriatic arthritis defined as ≥5 swollen joints (from a 66 swollen joint count [SJC]) and ≥5 tender joints (from a 68 tender joint count [TJC]) at Screening and Baseline (measurable dactylitis of a digit counts as a single swollen joint and if tender, then also a single tender joint).
• Have had a history of documented plaque psoriasis or currently active plaque psoriasis
• If using cDMARD therapy, subjects must have been on it for 12 weeks prior to screening, with a stable dose (including stable route of administration) for at least 4 weeks prior to baseline.
• If using non-drug therapies (including physical therapies), thse should be kept sable 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 cyclooxygenase-2 (COX-2) inhibitor.
• Have undergone surgical treatment for psoriatic arthritis including synovectomy and arthroplasty in more than 3 joints and/or within the last 12 weeks prior to screening
• Presence of very poor functional status or unable to perform self-care.
• Administration of a live or attenuated vaccine within 12 weeks prior to baseline