Intravesical Bacillus Calmette-Guérin (BCG) instillation is a widely used adjuvant therapy for non-muscle-invasive bladder cancer (NMIBC), typically associated with a favorable safety profile. However, severe complications such as systemic BCG infection, though rare, can occur and are potentially life-threatening. Diagnosing systemic BCG infection is challenging due to frequently negative microbiological results. We report the case of a 67-year-old immunocompetent man who developed a systemic BCG infection after undergoing BCG therapy for NMIBC. Initially misdiagnosed as a urinary tract infection, his condition worsened despite antibiotic treatment. The diagnosis of BCG sepsis was suspected after prolonged fever, asthenia, and other systemic symptoms. Prompt initiation of antituberculosis treatment with rifampicin, ethambutol, and isoniazid led to a full recovery. This case underscores the critical importance of early recognition and treatment of systemic BCG infections, particularly in patients presenting with persistent fever and no clear source of infection.