Purpose: This study analyzed the treatment effects of drain insertion in the posteromedial portal after an arthroscopic synovectomy and posterior septum resection for pyogenic arthritis. Materials and Methods: From June 2005 to December 2016, 56 cases (55 patients, 1 case of bilateral knee) were diagnosed with pyogenic arthritis and arthroscopic treatment was performed. Fourteen patients (25.0%) were identified with causative organisms. The average follow-up period was 12.3 months, and the mean age was 67.8 years old. Twenty-four cases were males and 31 cases were females. Kellgrene-Lawrence grade (K-L grade) I was 6 patients, II was 20 patients, III was 11 patients, and IV was 18 patients. Surgery was performed through six portal. The posterior septum was removed and a drainage tube was inserted from the posterior medial side to the posterior side of the joint. The Lysholm score was used after surgery for a functional evaluation. The K-L grade at the time of admission and at the last follow-up were compared for a radiological evaluation. Results: The mean normalization period of the C-reactive protein was 59.8 days (6-164 days). Intravenous antibiotics were used for an average of 37.1 days. The results of the functional evaluation revealed a mean Lysholm score of 64.5 (30-98) in a total of 56 cases. Two reoperations was performed, but the treatment was completed with same method in all patients. The K-L grade was similar at the time of surgery and at the last follow-up (p>0.05). Conclusion: The method of drain insertion in the posteromedial portal after arthroscopic synovectomy and posterior septum resection for pyogenic arthritis is an effective technique that can obtain satisfactory results by inducing proper exudate discharge.