There are a few reports on the treatment of nonunited tibial anterior cruciate ligament (ACL) avulsion fractures. To our knowledge, this is the first report of a case of nonunited tibial ACL avulsion fracture 30 years after injury. A 36-year-old woman injured her knee during a road traffic accident 30 years ago. Since then, she had persistent knee instability that was left untreated. She presented to our clinic because of knee pain she had been experiencing for a week. Radiography and computed tomography revealed tibial ACL avulsion fracture nonunions. Screw fixation with arthroscopy was performed, and bone fusion was obtained. The knee injury and osteoarthritis outcome score improved from 24 points preoperatively to 83 points postoperatively. The nonunion of intercondylar eminence fractures of the tibia, even up to 30 years after the initial injury, can be treated by osteosynthesis with arthroscopy.