Purpose: To investigate iris atrophy in Posner−Schlossman syndrome (PSS). Methods: Sixty-one patients with PSS were included. Using swept-source optical coherence tomography, the thickness of anterior border layer (A), middle stromal layer (M), and the posterior pigmented epithelial layer (P) of iris were measured at 500 μm, 1000 μm, 1500 μm, 2000 μm, and 2500 μm from the pupillary edge in both PSS-affected and fellow eyes. The relationships between iris thickness and corneal endothelium density, cup-to-disc (C/D) ratio, and retinal nerve fiber layer (RNFL) thickness were also estimated in PSS-affected eyes. Results: Iris thickness parameters, including M500, M1000, A1500, A2000, P2000, and P2500, were significantly thinner in PSS-affected eyes than in fellow eyes (all p < 0.05). Moreover, M500 and M1000 were significantly associated with corneal endothelium density (p = 0.047 and 0.018, respectively), and M500 was significantly associated with C/D ratio (p = 0.001) and RNFL thickness (p = 0.037) in PSS-affected eyes. Conclusions: Iris showed significant thinning and atrophy in PSS-affected eyes, and iris stromal thickness close to the pupillary edge could be a novel clinical predictor of the changes in corneal endothelium, C/D ratio, and RNFL thickness in PSS.