CASE: A 43-year-old former drug user presented with a mycobacterium-caused septic nonunion of the humeral shaft. The patient had undergone 2 previous surgeries for a hematogenous fracture-related infection due to Staphylococcus aureus. The procedures had included debridement, double-plate osteosynthesis, and placement of a gentamicin-loaded bone substitute. Culture specimens produced growth of Mycobacterium canariasense. Because the microbiological diagnosis was obtained after the patient had been discharged, no antibiotic treatment could be prescribed. Nevertheless, solid bone fusion was observed in the most recent outpatient examination. CONCLUSION: This case illustrates the importance of considering atypical pathogens in patients who are immunoincompetent. To the best of our knowledge, this is the first reported case of septic nonunion caused by M. canariasense.