Background: Arthroscopic capsulolabral repair for posterior shoulder instability has been shown to improve patient-reported outcomes and return to sport at short-term and midterm follow-up, but long-term outcomes are unknown. Purpose: To determine the objective and subjective clinical outcomes of shoulder function after arthroscopic posterior shoulder stabilization at a minimum 10-year follow-up. Study Design: Case series; Level of evidence, 4. Methods: A total of 53 patients (55 shoulders) with unidirectional recurrent posterior shoulder instability who underwent arthroscopic capsulolabral repair were evaluated at a mean follow-up of 15.4 years. Outcomes such as the American Shoulder and Elbow Surgeons (ASES) shoulder score, Kerlan-Jobe Orthopaedic Clinic shoulder and elbow score, and subjective strength, stability, range of motion, and pain were evaluated preoperatively and postoperatively at a minimum 10-year follow-up. Outcomes from the same patient cohort were previously collected at 3.0-year follow-up and were evaluated for longitudinal analysis. Subgroup analyses for sport type (contact vs noncontact), position (thrower vs nonthrower), and revision versus nonrevision were performed. Risk factors for revision surgery based on magnetic resonance imaging findings, patient characteristics, and surgical findings were investigated. Results: Arthroscopic capsulolabral repair for posterior shoulder instability led to a statistically significant improvement on all outcome measures, with similar values at short-term (3.0-year) and long-term (15.4-year) follow-up. Across the total population, patients with 19 of 55 shoulders (35%) returned to sport at the same preinjury level, and patients with 33 of 55 shoulders (60%) returned to sport at some level. At long-term follow-up, patients with only 22% of shoulders were involved in the primary sport in which the injury was sustained, with patients with 28% of shoulders discontinuing sporting participation because of ongoing shoulder issues. Throwers trended toward lower Kerlan-Jobe Orthopaedic Clinic scores than nonthrowers both preoperatively (36.5 ± 22.8 vs 48.7 ± 22.9, respectively; P = .10) and postoperatively (57.4 ± 27.0 vs 73.5 ± 26.8, respectively; P = .09) but had similar improvements. Contact and noncontact athletes had similar preoperative and postoperative values, with equal improvements after surgery. With failure defined as revision surgery, an ASES score <60, or a stability value >5, 19 of 55 shoulders (35%) met failure criteria at final follow-up. Overall, 7 of 55 shoulders (13%) underwent revision surgery. At long-term follow-up, patients who underwent revision surgery had worse outcomes than those who did not undergo revision (ASES score: 53.1 ± 25.9 vs 81.8 ± 19.4, respectively; P < .001). An acute injury in the postoperative period ( P < .001) and a smaller glenoid bone width on magnetic resonance imaging ( P = .02) were the only identified risk factors for revision surgery. Conclusion: Arthroscopic capsulolabral repair for posterior shoulder instability was a durable treatment option that improved long-term shoulder pain and function and facilitated return to sport in the majority of patients at a mean follow-up of 15.4 years, although a notable proportion of patients met various criteria for failure.