The mechanical properties of an ankle-foot orthosis (AFO) can impact how a user's movement is either restricted or augmented by the device. However, standardized methods for assessing stiffness properties of AFOs are lacking, posing a challenge for comparing between devices and across vendors. Therefore, the purpose of this study was to quantify the rotational stiffness of thirteen commercial, non-articulated, carbon composite ankle-foot orthoses. A custom, instrumented test fixture deflected an AFO through 20 degrees of plantar/dorsiflexion motion about a specified, but adjustable, ankle axis. Sagittal, frontal, and transverse plane rotational stiffness were calculated, and reliability was assessed between cycles, sessions, and testers. The test fixture demonstrated good-to-excellent reliability between testers, sessions, and cycles (intraclass correlation coefficients all ≥0.95 for sagittal plane stiffness measures). Sagittal plane AFO stiffness ranged from 0.58 Nm/deg to 3.66 Nm/deg. AFO's with a lateral strut demonstrated frontal plane stiffnesses up to 0.71 Nm/deg of eversion while those with a medial strut demonstrated frontal plane stiffnesses up to 0.53 Nm/deg of inversion. Transverse plane stiffnesses were less than 0.3 Nm/deg of internal or external rotation. These results directly compare AFOs of different models and from different manufacturers using consistent methodology and are intended as resource for clinicians in identifying a device with stiffness properties for individual patients.