Osteosarcoma is the most common primary malignant bone tumor, especially in younger patients. Diagnosis is based on the combined evaluation of radiological, clinical, and pathological examinations. It is usually located in the distal femur, proximal tibia, and proximal humerus. The fibula is a rare localization for osteosarcoma. Surgery in this region is challenging due to the complex anatomic structures around the knee. Especially the peroneal nerve, lateral collateral ligament (LCL), and popliteal vessel branches are of critical importance. However, additional structures such as the arcuate ligament, biceps femoris, and iliotibial band play an essential role in the stabilization of the knee. Thus, these structures must be protected as much as possible. This case report aims to present the diagnosis and treatment process of conventional osteosarcoma in the proximal fibula, which was located close to the peroneal nerve and required LCL reconstruction after the resection.