OpenCap, a smartphone-based markerless system, offers a cost-effective alternative to traditional marker-based systems for gait analysis. However, its kinematic measurement accuracy must be evaluated before widespread use in clinical practice. This study aimed to evaluate OpenCap for lower-limb joint angle measurements during walking in patients with knee osteoarthritis (OA) and to compare error metrics between patients and healthy controls. Lower-limb kinematic data were simultaneously collected from 53 patients with knee OA and 30 healthy individuals using OpenCap and a marker-based motion capture system while walking at a self-selected speed. Evaluation was assessed through root mean square error (RMSE) and intraclass correlation coefficient (ICC). Two-way repeated measures analyses of variance were employed to evaluate the main effects of and interactions between group (knee OA patients vs. healthy controls) and walking direction (toward vs. away from the camera). The results demonstrated a grand mean RMSE of 6.1° and an ICC of 0.67 for knee OA patients when walking toward the camera. Patients with knee OA exhibited significantly higher RMSE and lower ICC values compared to healthy controls. Additionally, walking toward the camera was associated with significantly lower RMSE and higher ICC values than walking away from the camera. OpenCap’s minimal hardware costs, free software, and user-friendly interface suggest its potential for widespread clinical implementation. The sagittal hip and knee angles demonstrate strong agreement with the marker-based system;​ however, caution is warranted in clinical decision-making for this population, as errors in most joint angles slightly surpass acceptable thresholds.