Background: Ankylosing spondylitis (AS) is a chronic inflammatory disease affecting the axial skeleton, with systemic symptoms such as fever being uncommon. Adult-onset Still’s disease (AOSD) is an autoinflammatory condition characterized by high-grade fever, rash, and polyarthritis. The co-occurrence of AS and AOSD is extremely rare and presents significant diagnostic challenges. This case report outlines the complexities involved in identifying and managing such an overlap. Case Presentation: A 37-year-old male with a history of AS presented with pyrexia of unknown origin (PUO), bilateral asymmetric polyarthritis, and a transient skin rash. Laboratory findings showed elevated inflammatory markers (CRP: 322 mg/L, ESR: 82 mm/hr), hyperferritinemia (>10,000 ng/mL), hyperleukocytosis (16,000/mm³) with neutrophil predominance, and normocytic normochromic anemia (hemoglobin: 9 g/dL). Extensive infectious and immunological investigations were negative, while PET CT revealed metabolically active lymphadenopathy. Biopsies excluded malignancy and macrophage activation syndrome. A diagnosis of AOSD was established based on Fautrel and Yamaguchi criteria. The patient’s treatment was shifted from NSAIDs to corticosteroids (prednisone 1 mg/kg/day), leading to significant improvement, followed by maintenance therapy with DMARDs to sustain remission. Conclusion: This case highlights the importance of considering AOSD in AS patients presenting with PUO and systemic inflammation. Timely diagnosis and appropriate treatment can reduce complications. Further studies are needed to explore the pathophysiological links and optimize management strategies.