Mycobacterium terrae is ubiquitous in our environment. M terrae infections most commonly involve tendon sheaths, bones, bursae, and joints. We report a case of infectious arthritis of the knee caused by M terrae in a 21-year-old man who had non-specific chronic synovitis. No organism was seen on microscopy or isolated from cultures until months later. Initially the M terrae culture was considered a contaminant and specific anti-mycobacterial treatment was not advised. The patient was commenced on suppressive therapy for persistent effusion and discomfort. Eventually, the M terrae infection was confirmed and he was commenced on clarithromycin, ciprofloxacin, and ethambutol. The triple antibiotic regimen was continued for 2 years. The knee improved but never completely settled. The patient chose to cease all antibiotic medication. The knee remained swollen and irritable, with little chance of eradicating the organism.