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).