Adult-onset Still's disease (AOSD) is a systemic inflammatory disease characterized by rash, arthritis, and persistent spiking fever. The diagnosis and treatment of AOSD are challenging due to the lack of specific diagnostic criteria and little evidence of effective treatments. Here, we reported a case of an 18-year-old woman with a fever of unknown origin (FUO), evanescent rash (without the typical “salmon-pink” color), and systemic lymphadenopathy. Laboratory tests at hospital admission revealed marked hyperferritinemia of 12,100 ng/mL. AOSD was subsequently suspected. Additional anti-nuclear-antibody analysis for differential diagnosis was negative. The initiation treatment with high-dose prednisolone, tapered to half every week, was immediately started. The symptoms temporarily improved but relapsed during the tapering period. The prednisolone dose was increased again, and tocilizumab was introduced. Symptom remission and prednisolone dose reduction were subsequently achieved. Therefore, a medication tapering schedule and treatment replacement to inhibit the pathophysiology of AOSD need to be carefully considered. While a ferritin test is useful to diagnose AOSD based on the presence of FUO, there are AOSD patients without hyperferritinemia. Additionally, AOSD rash on Asian skin may not present with the typical “salmon-pink” color.