Knee pain is a common clinical complaint with a broad differential diagnosis. In critically ill patients, acute monoarticular pain and swelling typically raise concern for septic arthritis. However, alternative etiologies such as crystal-induced arthritis, trauma, and hemarthrosis must also be considered.

This report presents a rare case of spontaneous hemarthrosis in a patient receiving prophylactic enoxaparin. Although uncommon, spontaneous hemarthrosis is a significant complication of anticoagulation therapy, particularly with low molecular weight heparins (LMWH) like enoxaparin.