Purpose To examine the relationship of choroidal thickness with axial length (AL) and myopia in young adult eyes in the ethnically diverse Correction of Myopia Evaluation Trial (COMET) cohort. Design Cross-sectional, multicenter study. Methods In addition to measures of myopia by cycloplegic autorefraction and AL by A-scan ultrasonography, participants underwent optical coherence tomography imaging of the choroid in both eyes at their last visit (14 years after baseline). Using digital calipers, 2 independent readers measured choroidal thickness in the right eye (left eye if poor quality; n = 37) at 7 locations: fovea and 750, 1500, and 2250 μm nasal (N) and temporal (T) to the fovea. Results Choroidal thickness measurements were available from 294 of 346 (85%) imaged participants (mean age: 24.3 ± 1.4 years; 44.9% male) with mean myopia of -5.3 ± 2.0 diopters and mean AL of 25.5 ± 1.0 mm. Overall, choroidal thickness varied by location (P <.0001) and was thickest at the fovea (273.8 ± 70.9 μm) and thinnest nasally (N2250, 191.5 ± 69.3 μm). Multivariable analyses showed significantly thinner choroids in eyes with more myopia and longer AL at all locations except T2250 (P ≤.001) and presence of peripapillary crescent at all locations except T1500 and T2250 (P ≤.0001). Choroidal thickness varied by ethnicity at N2250 (P <.0001), with Asians having the thinnest and African Americans the thickest choroids. Conclusion Choroids are thinner in longer, more myopic young adult eyes. The thinning was most prominent nasally and in eyes with a crescent. In the furthest nasal location, ethnicity was associated with choroidal thickness. The findings suggest that choroidal thickness should be evaluated, especially in the nasal regions where myopic degenerations are most commonly seen clinically.