Engraftment Syndrome (ES) is a non-infectious complication occurring during the peri-engraftment period of hematopoietic stem cell transplant (HSCT). We present a case of a 9-year-old boy with relapsed Hodgkin lymphoma who underwent autologous HSCT and developed early-onset ES with predominant gastrointestinal and cardiogenic manifestations. The onset of ES on Day +5 post-HSCT, prior to neutrophil recovery and in absence of classical features represents an unusual presentation. Prior nivolumab exposure possibly primed the immune system, leading to exaggerated cytokine response. Timely administration of corticosteroids resulted in rapid clinical recovery. This case underscores the need for heightened vigilance for atypical presentations of ES.