We report a rare case of atypical mycobacteriosis diagnosed by 18F-FDG PET/CT. A 28-year-old man with a history of kidney transplantation presented with fever, painful nodular skin lesions, and elevated inflammatory markers. FDG PET/CT demonstrated multiple subcutaneous, cutaneous, and osseous areas of increased tracer uptake. Histopathologic and microbiological evaluation revealed disseminated infection with Mycobacterium haemophilum. FDG PET/CT is a valuable tool for early and correct diagnosis of occult sources of infection in immunocompromised patients.