The precise pathogenesis of COVID-19-related multisystem inflammatory syndrome remains largely elusive, despite its rarity. The syndrome symptoms often overlap with those of other infections, posing challenges for prompt diagnosis. A male patient, 34 years old, was admitted with suspicion of severe dengue, rapidly progressing to multiple organ dysfunction. Dengue tests resulted negative, and he passed away after four days. This case occurred approximately four weeks after the initial onset of COVID-19 and met all diagnostic criteria as defined by the Centers for Disease Control and Prevention. This report presents the first documented case of fatal multisystem inflammatory syndrome in adult (MIS-A) in Brazil. Recognizing the significance of suspecting this syndrome and promptly initiating treatment at an early stage are essential for minimizing damage and mortality.