<p>Accurate assessment of three-dimensional ankle kinematics is essential for advancing biomechanical analyses and informing potential clinical applications. While optical motion capture systems (OMCs) are considered the gold standard, their high cost and limited portability restrict their use outside the laboratory. Inertial measurement units (IMUs) offer a more practical alternative; however, concerns remain regarding their validity and reliability in diverse walking conditions. This study evaluated the agreement and repeatability of IMU-derived ankle kinematics relative to OMCs across three walking surfaces: level ground, inward wedge, and outward wedge. Ankle movements in the sagittal, frontal, and transverse planes were analyzed using a minimal sensor configuration. Moderate to high agreement and consistent repeatability were observed in the sagittal and transverse planes, particularly during level and inward wedge floor walking. Conversely, the frontal plane demonstrated limited agreement under wedged conditions, likely because of complex multisegmental foot dynamics and reference frame misalignment from static calibration. Despite these limitations, consistent repeatability across conditions supports the use of IMUs for tracking ankle kinematics outside the laboratory. These findings suggest the preliminary applicability of IMUs as a functional tool for interpreting ankle kinematics outside the laboratory.</p>