Background: Open surgical excision of the os trigonum has been the traditional treatment for posterior ankle impingement syndrome (PAIS). However, the endoscopic excision has recently become quite popular. Purpose/Hypothesis: The purpose of our study was to compare the results of endoscopic versus open excision of a symptomatic os trigonum for the treatment of PAIS in an athletic population. It was hypothesized that the endoscopic technique would be superior to the open technique regarding functional outcomes, pain, and time to return to training and the previous sports level. Study Design: Randomized controlled trial; Level of evidence, 2. Methods: From 2008 to 2011, 52 athletes underwent a symptomatic os trigonum excision; 26 athletes had an open procedure (group A) and 26 had an endoscopic procedure (group B). The American Orthopaedic Foot and Ankle Society (AOFAS) hindfoot score and the Visual Analog Score-Foot and Ankle (VAS-FA) were obtained, and the time to return to training and to previous sports level was recorded. Results: Patients in group B appeared to have significant improvement of AOFAS hindfoot score compared with those in group A (P < .05), whereas no statistical significance was found for the postoperative VAS-FA scores between the 2 groups. The mean SD time to return to training was 9.58 +/- 3.98 weeks for group A and 4.58 +/- 1.47 weeks for group B (P < .001). The time to return to previous sports level was 11.54 +/- 3.89 weeks for group A and 7.12 +/- 2.25 weeks for group B (P < .001). The overall complication rate was 23% for group A (6 cases) and 3.8% for group B (1 case). Conclusion: Both the open procedure and the endoscopic approach yielded acceptable outcomes in terms of function and pain. However, complication rates were remarkably lower with endoscopic treatment, and the time to return to full activities was much shorter. Endoscopic excision of the os trigonum is a safe and effective treatment option for athletes who require early return to their previous sports level.