ABSTRACT Background Chronic postâthrombotic syndrome (PTS) involving the peroneal venae comitantes is an uncommon but potentially flapâthreatening finding during fibular free flap reconstruction that may not be detected on routine preoperative computed tomography angiography. We describe the successful management of extensive chronic PTS encountered unexpectedly during fibular free flap reconstruction for advanced oral cavity squamous cell carcinoma. Methods Following unsuccessful mechanical thrombectomy, arterial inflow was established before venous reconstruction to identify the dominant functional venous outflow pathway, followed by selective venous anastomosis. Results Early postoperative venous congestion was successfully managed with revision and a second venous anastomosis. The flap remained viable with excellent functional and oncologic outcomes at 1 year. Conclusion This case demonstrates that chronic PTS should not necessarily preclude fibular free flap reconstruction and presents a practical intraoperative and postoperative strategy for managing this challenging and previously underreported scenario.