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Human gait is a significant health indicator used in a wide range of applications, including diagnosis, monitoring, rehabilitation, and sport. Among all the human joints movements, the knee flexion/extension is a crucial movement, and its evaluation may provide valuable information related to pathologies, walking disorders, to evaluate the efficiency of prescribed therapies and the rehabilitation of patients, and to improve performances of athletes. In this work, we designed, fabricated, and tested a wearable system (i.e., an instrumented knee brace) for monitoring knee movements under static and dynamic conditions. The system consists of a single fiber Bragg grating (FBG) sensor encapsulated in a soft and flexible polymer matrix in contact with the knee brace. The metrological characterization of two sensors (dimensions 50 mm × 30mm × 10mm and 40mm × 30mm × 10mm) has been performed by applying strains up to 2% which correspond to max length variations (ΔL MAX ) of 0.76 mm and 0.55 mm for the big and the small sensors, respectively. Additionally, pilot trials have been carried to preliminary test the instrumented knee brace for monitoring flexion/extension movements during walking at different speeds (i.e., 3 km/h, 4 km/h, 5 km/h). Both the sensors showed an approximately linear response in the range of interest and sensitivities up to 3.944 nm-mm−1, The wearable system was able to monitor the number of gait cycles and to discriminate the knee flexion and extension movements during walking.