This study aimed to determine the safety and efficacy of tramadol 37.5 mg/acetaminophen 325 mg combination tablets (Ultracet®) in patients with ankylosing spondylitis (AS). This was a 12-week, randomized, double-blind, placebo-controlled study. Sixty patients with active AS according to the Modified New York Criteria were enrolled. Active disease was defined by Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) for more than 3 at randomization. Subjects were randomized equally into two groups: the treatment group received aceclofenac plus Ultracet® one tablet twice a day, and the control group received aceclofenac plus placebo for 12 weeks. The primary endpoint was a difference of Assessment in Ankylosing Spondylitis (ASAS20) response criteria between two groups at week 12. At week 12, ASAS20 was achieved by 53.3 % of the aceclofenac plus Ultracet group and 31 % of the aceclofenac alone group (p = 0.047). For the pain visual analogue scale at week 12, there was a reduction of 45.6 % in aceclofenac plus Ultracet group and 25.7 % in the aceclofenac alone group (p = 0.087). There was no statistically significant difference between two groups in BASDAI, Bath Ankylosing Spondylitis Functional Index, Bath Ankylosing Spondylitis Global Index, Physician Global Assessment, spinal mobility, ESR, hs-CRP, and Ankylosing Spondylitis Quality of Life Questionnaire. A slight increase in total adverse events was noted with dizziness (7.5 vs 1.5 %), vertigo (4.5 vs 1.5 %), and nausea/vomiting (6 vs 0 %) in the Ultracet arm compared to placebo. The tramadol 37.5 mg/acetaminophen 325 mg combination tablet (Ultracet®) might has additional effect to nonsteroidal anti-inflammatory drugs in the treatment of patients with ankylosing spondylitis. It showed marginal benefit in pain and disease activity. However, a slight increase in minor adverse events was noted.