BACKGROUND/AIMS: To develop, test and determine whether a surgical-competency assessment tool for simulated glaucoma surgery is valid. METHODS: The trabeculectomy ophthalmic simulated surgical competency assessment rubric (Sim-OSSCAR) was assessed for face and content validity with a large international group of expert eye surgeons. Cohorts of novice and competent surgeons were invited to perform anonymised simulation trabeculectomy surgery, which was marked using the Sim-OSSCAR in a masked fashion by a panel of four expert surgeons. Construct validity was assessed using a Wilcoxon rank-sum test. Krippendorff's alpha was calculated for interobserver reliability. RESULTS: For the Sim-OSSCAR for trabeculectomy, 58 of 67 surgeons (86.6%) either agreed or strongly agreed that the Sim-OSSCAR is an appropriate way to assess trainees' surgical skill. Face validity was rated as 4.04 (out of 5.00). Fifty-seven of 71 surgeons (80.3%) either agreed or strongly agreed that the Sim-OSSCAR contents represented the surgical technique of surgical trabeculectomy. Content validity was rated as 4.00. Wilcoxon rank-sum test showed that competent surgeons perform better than novices (p=0.02). Interobserver reliability was rated >0.60 (Krippendorff's alpha) in 19 of 20 steps of the Sim-OSSCAR. CONCLUSION: The Sim-OSSCAR for trabeculectomy, a newly developed and validated assessment tool for simulation glaucoma surgery, has validity and reliability. It has the potential to play a useful role in ophthalmic surgical education.