Measurements for the functional evaluation of knee joint mobility are essential, e.g., to determine the need for invasive surgery and quantify patient outcomes. While vision-based motion capture systems offer high precision, they require specialized laboratories and trained staff. This paper introduces KARMA (Knee Angle Resistive Measurement Apparatus), an instrumented knee sleeve with six resistive flex sensors to measure knee joint angles. The proposed device features wearability, low cost, and flexibility, enabling pre- and post-operative evaluations for patients eligible for knee surgery. A case study was performed using a marker-based motion capture system as a reference. This demonstrated KARMA’s ability to measure flexion-extension angles but highlighted its inability to measure abduction-adduction and internal-external rotation angles. We discuss the potential of this novel approach and propose subsequent design improvements.