Abstract Purpose The relationship between ciliary muscle thickness (CMT), age and refractive error was investigated to determine if CMT, like other anterior ocular anatomy, differs in adults with Down syndrome (DS). Methods The CMT of 33 adults with DS was imaged using anterior segment optical coherence tomography. Images from the right eye obtained 45 minutes after cycloplegia (1% tropicamide, 2.5% phenylephrine) were analysed to calculate thickness at 1, 2 and 3 mm posterior to the scleral spur (CMT1, CMT2, CMT3), maximum thickness (CMTMAX) and apical thickness (AT = CMT1 – CMT2). Spherical equivalent refractive error was determined by clinical refraction using both non‐dilated and dilated measures. Multivariate regression analysis evaluated the relationship between CMT and refractive error while controlling for subject age. Results Images were analysed from 26 subjects (mean age (SD) 29 years; mean refractive error (SD): −0.90 (5.03) D, range: −15.75 to +5.13D). Mean (SD) CMT decreased with posterior position (CMT1: 804 (83) μm; CMT2: 543 (131) μm; CMT3: 312 (100) μm). Mean (SD) CMTMAX and AT was 869 (57) μm and 260 (84) μm, respectively. There was a significant linear correlation indicating thinning CMT with increasing age for CMT1 and CMT2 (p ≤0.05). CMT2 and CMT3 had a significant negative correlation (thicker muscle with increasing myopic refractive error) (p ≤0.01). AT had a significant positive correlation (thicker muscle with increasing hyperopic refractive error) (p <0.01). Conclusions Ciliary muscle thickness in participants with DS was found to be in a similar range with similar refractive error trends to previous reports of individuals without DS. However, it is important to note that the refractive error trends were driven by individuals with moderate to high levels of myopia.