A 10-year-old Labrador Retriever was referred for persistent lameness due to chronic right shoulder pain, unresponsive to various pain management therapies. Radiographs indicated signs of severe degenerative changes in the joint. Synovial fluid analysis was not diagnostic. Septic arthritis was suspected based on computed tomography and clinical signs. Arthroscopy allowed joint exploration, tissue biopsies, and copious joint lavage. Trichophyton spp. proliferation was identified on antimicrobial culture and histological analysis on samples obtained during arthroscopy. Oral griseofulvin therapy was initiated. Two months later, the referring veterinarian decided to interrupt the treatment after a negative synovial culture despite persistent lameness. Euthanasia was elected upon after pain also appeared on the tarsus; a post-mortem exam was not authorized by the owner. The origin of the infection remains unclear as this patient had no skin lesions and its immunological status was unknown. However, dermatophytosis has been reported in healthy dogs without skin lesions. To the author's knowledge, this is the first report of an osteoarticular infection with a dermatophyte in a dog.