Abstract Introduction: Narrow anterior chamber angles (ACAs) are a risk factor for closed-angle glaucoma development, being highly prevalent in Latin-American population. Manual gonioscopy remains the gold standard for ACA evaluation; however, new technologies have emerged, mainly to improve measurement objectivity. Purpose: The purpose of the study was to estimate the agreement and reproducibility of ACA measurement between manual gonioscopy performed with Posner lens, versus semi-automated gonioscopy using a prototype of the GS-1 gonioscope (NIDEK Co., Ltd.). Materials and Methods: This was a prospective blinded diagnostic test and interobserver agreement study. Manual gonioscopy was performed by a glaucoma specialist (observer 1), and semi-automated gonioscopy was taken by trained technicians using the GS-1 device. Obtained images were saved and later analyzed by a second glaucoma specialist (observer 2). Univariate analysis and Cohen’s kappa coefficient were calculated to estimate agreement between the two methods for ACA amplitude measurement, graded according to the Shaffer’s classification system. Results: One thousand and five eyes of 503 patients were included in the study. The mean age was 50.1 years (range 18–93 years) and 64.5% of participants were women. Cohen’s kappa coefficient showed a moderate agreement among the two observers for ACA amplitude evaluation, which translated into a moderate reproducibility for gonioscopy performed with the GS-1 device compared with the gold standard ( P < 0.05). Conclusion: Angle evaluation with semi-automated devices, such as the GS-1 gonioscope, can demonstrate concordant information compared with the gold standard, which remains being manual gonioscopy. Its main advantage is the possibility of storing photographic records for patient’s follow-up over time; however, these devices present certain limitations that make it necessary to carry out further studies.