Accounting for 2.2-4.7% of all tuberculosis cases in Europe and USA and around 10-15% of extra-pulmonary tuberculosis cases, osteoarticular tuberculosis tends to be chronic, slowly progressive and destructive. We report the case of an 81-year-old male with 3 weeks of progressively worsening pain, swelling and limited range of motion of the left knee. A knee arthroscopy was performed for synovial biopsy at our department revealing diffuse synovitis with scarce villi formation. The positive polymerase chain reaction assay for Mycobacterium tuberculosis in the synovial tissue allowed the establishment of the diagnosis and synovium histology showed caseating granulomas. A lengthy delay between first symptoms of osteoarticular tuberculosis and the beginning of treatment has been reported. A high index of suspicion, synovial membrane biopsy and appropriate microbiologic testing are fundamental to avoid a delay in diagnosis.