ntroduction: Diffuse pigmented villonodularsynovitis(PVNS) of the knee is a locally destructive lesion that tendsto recur following surgical synovectomy. This study aims to evaluate the long-term functional outcomes and recurrence rates of combined staged synovectomy and external radiotherapy in managing diffuse PVNS of the knee. Methods: The data of twenty-three patients who had a diffuse PVNS of the knee between June 2011 and September 2017 were retrospectively collected. The patients underwent combined staged anterior arthroscopic and open posteriorsynovectomy followed by low-dose externalradiotherapy (average 3000 cGy). Functional outcomes were assessed preoperatively and at the final follow-up using the International Knee Documentation Committee (IKDC) score, Musculoskeletal Tumor Society (MSTS) score, Western Ontario and McMaster Universities Osteoarthritis (WOMAC) score, and SF-36 score, along with the knee motion measurements. Statistical analysis evaluated the changes in functional scores and recurrence rates, with a p-value < 0.05 as the cut-off level of significance. Results: The mean follow-up time was 121.1 ± 20.2 months, with a minimum of 85 months. At the final followup, the mean range of motion (ROM) improved from 84.1 ± 16.2◦ to 110.4 ± 9.7◦. The WOMAC score increased significantly from 44.5 ± 5.1 to 81.5 ± 6.3 (p < 0.00001). The IKDC score improved from 43.7 ± 8.1 to 79.1 ± 7.4 (p < 0.00001), MSTS scores from 6.7 ± 2.1 to 20.3 ± 5.7 (p < 0.00001), and SF-36 scores from 28.6 ± 7.4 to 77.4 ± 11.5 (p < 0.00001). Two recurrent cases were documented at the last follow-up visit (8.6%), with a minor complication rate of 21.7%. Conclusion: Combined staged synovectomy and external radiotherapy is an effective treatment for diffuse PVNS of the knee. It has demonstrated significant functional improvement and low recurrence rates over extended follow-up periods, supporting its role as a durable and reliable treatment strategy. Level of evidence: Level IV, retrospective cohort