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Disseminated Bacillus Calmette-Guérin (BCG) infection is a rare complication of intravesical BCG therapy, affecting both immunocompetent and immunocompromised patients. We present a case of disseminated BCG infection in an 86-year-old man undergoing immune checkpoint inhibitor therapy for relapsed Hodgkin lymphoma and concomitant intravesical BCG therapy for non-muscle-invasive bladder cancer. The patient developed a progressive mycotic abdominal aortic aneurysm, psoas abscess, and vertebral osteomyelitis. Mycobacterium bovis BCG was confirmed via Polymerase Chain Reaction (PCR) and mycobacterial culture from a psoas abscess biopsy. Despite appropriate antimycobacterial therapy and multiple attempts at abscess drainage, his condition deteriorated, ultimately leading to his death. This case highlights the challenges of diagnosing disseminated BCG infection, which can mimic malignancy, and suggests a possible link between immune checkpoint inhibitors and BCG dissemination. Clinicians should maintain a high index of suspicion for BCG-related complications, particularly among those receiving immune-modulating therapies.