Mpox (monkeypox) is a zoonotic disease that has been endemic in African countries for decades, with a recent outbreak in several countries around the world. A 39-year-old male with HIV-HBV coinfection and poor adherence to antiretroviral treatment, who was severely immunocompromised and had a concurrent diagnosis of Mpox infection, presented to our hospital with disseminated dermatosis (over 350 lesions), perianal ulcers, odynophagia, oral intolerance, diarrhea, and soft-tissue bacterial superinfection of the lower extremities. Laboratory results were consistent with HBV infection, with an absolute CD4 cell count of 40 cells/uL and a positive PCR result for Mpox. An abdominopelvic CT scan showed evidence of severe proctitis and perineal soft-tissue infection. After 65 days of Mpox PCR, new lesions in the vesicular stage continued to appear, eventually developing hemodynamic instability and sepsis, resulting in a fatal outcome. Our case highlights the importance of intentionally looking for risk factors such as HIV/HBV coinfection and evaluating immune status (CD4 cell count) in patients with severe Mpox infection because it could be related to higher mortality.