A Study Evaluating the Safety and Efficacy of Upadacitinib in Adults With Active Ankylosing Spondylitis

* Participant with a clinical diagnosis of ankylosing spondylitis (AS) and meeting the modified New York criteria for AS. * Participant must have baseline disease activity as defined by having a Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score \>= 4 and a Patient's Assessment of Total Back Pain score \>= 4 based on a 0 - 10 numeric rating scale (NRS) at the Screening and Baseline visits. * Participant has had an inadequate response to at least two nonsteroidal anti-inflammatory drugs (NSAIDs) over an at least 4-week period in total at maximum recommended or tolerated doses, or participant has an intolerance to or contraindication for NSAIDs as defined by the Investigator. * If entering the study on concomitant methotrexate (MTX), leflunomide, sulfasalazine (SSZ), and/or hydroxychloroquine, participant must be on a stable dose of MTX (\<= 25 mg/week) and/or SSZ (\<= 3 g/day) and/or hydroxychloroquine (\<= 400 mg/day) or leflunomide (\<= 20 mg/day) for at least 28 days prior to the Baseline visit. A combination of up to two background conventional synthetic disease-modifying anti-rheumatic drugs (csDMARDs) is allowed except the combination of MTX and leflunomide. * If entering the study on concomitant oral corticosteroids, participant must be on a stable dose of prednisone (\<= 10 mg/day), or oral corticosteroid equivalents, for at least 14 days prior to the Baseline visit. * If entering the study on concomitant NSAIDs, tramadol, combination of acetaminophen and codeine or hydrocodone, and/or non-opioid analgesics, participant must be on stable dose(s) for at least 14 days prior to the Baseline visit.

* Prior exposure to any Janus kinase (JAK) inhibitor (including but not limited to tofacitinib, baricitinib, and filgotinib). * Prior exposure to any biologic therapy with a potential therapeutic impact on spondyloarthritis (SpA). * Intra-articular joint injections, spinal/paraspinal injection(s), or parenteral administration of corticosteroids within 28 days prior to the Baseline visit. Inhaled or topical corticosteroids are allowed. * Participant on any other DMARDs (other than those allowed), thalidomide or apremilast within 28 days or five half-lives (whichever is longer) of the drug prior to the Baseline visit. * Participant on opioid analgesics (except for combination acetaminophen/codeine or acetaminophen/hydrocodone which are allowed) or use of inhaled marijuana within 14 days prior to the Baseline visit. * Participant has a history of inflammatory arthritis of different etiology other than axial SpA (including but not limited to rheumatoid arthritis, psoriatic arthritis, mixed connective tissue disease, systemic lupus erythematosus, reactive arthritis, scleroderma, polymyositis, dermatomyositis, fibromyalgia), or any arthritis with onset prior to 17 years of age. * Laboratory values meeting the following criteria within the Screening period prior to the first dose of study drug: serum aspartate transaminase \> 2 × upper limit of normal (ULN); serum alanine transaminase \> 2 × ULN; estimated glomerular filtration rate by simplified 4-variable Modification of Diet in Renal Disease formula \< 40 milliliter (mL)/minute/1.73m\^2; hemoglobin \< 10 gram/deciliter, total white blood cell count \< 2,500/microliter (μL); absolute neutrophil count \< 1,500/μL; absolute lymphocyte count \< 800/μL; and platelet count \< 100,000/μL.