With emergent Sars-Cov-2, a highly transmissive virus that caused millions of deaths worldwide, the development of vaccines became urgent to combat COVID-19. Although rare, important adverse effects had been described in a hypothetical scenario of immune system overstimulation or overreaction. Still's disease is a rare inflammatory syndrome of unknown etiology. It manifests as a cytokine storm, mainly IL-18 and IL-1β, and presents itself with fever spikes, joint pain, maculopapular evanescent salmon-pink skin rash, and sore throat, among other symptoms. Here, we report a case of a 44-year-old healthy male who developed adult-onset Still's disease (AOSD) with atypical symptoms after both doses of ChAdOx1 nCoV-19 vaccine with 3 months of dose interval. The medical team suspected Still's disease and started prednisone 1 mg/kg (40mg). The next day the patient showed a marked improvement in articular and chest pains and had no other fever episodes. Therefore, he was discharged to continue the treatment in outpatient care. On the six-month follow-up, the patient was free of complaints, and the progressive corticoid withdrawal plan was already finished.