We present the case of an adolescent male developing an ankylosis of the knee after septic arthritis following anterior cruciate ligament reconstruction (ACLR). The patient was shifted to our institution with postoperative septic arthritis associated with a systemic septic condition. Before, repeated arthroscopic surgery had been conducted without any improvement. MRI showed a concomitant osteomyelitis. The infection (Gächter IV, S. aureus) was controlled by an open surgical approach and graft removal. An increasing joint stiffness was documented. X-rays showed an ankylosis at 30° of flexion and early closure of growth plates. Functional knee scores showed significantly worse results. Early diagnosis and a stage-adapted treatment in septic arthritis following ACLR are mandatory. In advanced stages or concomitant osteomyelitis, an open approach and graft removal may be appropriate. The antibiotic treatment should be adapted consistently.