AbstractMycobacterium marinum (M. marinum) is a slowly growing nontuberculous mycobacterium. The incidence of M. marinum infections in Denmark is unknown. We conducted a retrospective nationwide study including all culture confirmed cases of M. marinum from 2004 to 2017 in Denmark. All available medical records were reviewed. Demographics, clinical characteristics, and treatment regiments were analyzed. Fifty-five patients were identified, 40 (72.7%) were men with a median age of 50 years. Aquatic exposure was reported by 48 (90.6%) of the patients. Site of infection was upper extremities in 49 (92.5%) patients and 49 (92.5%) had superficial infection. The median time from symptom presentation to diagnosis was 194 days. All patients received antibiotics. Median time of treatment duration among all patients was 112 days. Treatment outcome was classified as improved in 40 (75%), improved with sequela in 4 (7.6%) patients and only 3 patients (3.8%) were classified as failed. Infection with M. marinum is rare and there is a long delay from symptom manifestation to diagnosis. The infection is predominantly related to aquatic exposure. M. marinum should be a differential diagnose in patients with slow-developing cutaneous elements and relevant exposure. Treatment outcomes are overall good and severe sequela are rare.