Moraxella catarrhalis was isolated from blood from a 41-year-old man who had a 24-hour history of increasing pain in and swelling of the left knee. No history of trauma, arthropathy, fever, chills, cough, or chest pain was noted. What is believed to be the first case of bacteremia caused by M. catarrhalis that was associated with septic arthritis is described in this report. The case presented suggests the pathophysiology of this rare condition. One previous case of septic arthritis caused by M. catarrhalis without documented bacteremia has been reported.