Purpose To investigate the iris thickness (IT) of patients with primary open-angle glaucoma (POAG) and older adults using anterior segment swept-source optical coherence tomography (SS-OCT). Methods In this comparative cross-sectional study, 154 participants were enrolled, including 40 patients with POAG and 114 healthy individuals. Nasal-angle SS-OCT images were analysed using callipers to measure the thickness of the iris, including the anterior border layer, stromal, and pigmented epithelial layer, at 1 and 2 mm from the pupil edge. The relationship between IT and glaucoma severity was analysed, and receiver operating characteristic curves were constructed to assess the ability of each IT parameter to distinguish patients with glaucoma from healthy controls. Results The IT parameters and iris area were lower in the POAG group than in the age-matched control group (p < 0.05). In the POAG group, the thickness of the pigmented epithelial layer at 1 and 2 mm was significantly correlated with the severity of glaucoma (p < 0.05). Iris area, with a cut-off of 1.43 mm(,)(2) exhibited the highest sensitivity (85%) and specificity (64.81%; area under the curve = 0.783) for distinguishing patients with POAG from healthy controls. Older adults tended to have the thinnest IT, children had the smallest iris area (p < 0.05). Conclusions In patients with POAG, the severity of iris atrophy was associated with glaucoma severity. The results of IT measurements by SS-OCT may help in the clinical evaluation of POAG.