This paper presents the development of a low-cost, fully portable, wearable sensor for joint angle assessment based on a polymer optical fiber (POF) curvature sensor. The mechanical support configurations as well as the fiber length are analyzed to obtain a sensor with lower hysteresis and higher sensitivity and linearity. In addition, the annealing is made in the fiber to further reduce the sensor errors, and an analysis to obtain the sensor cross-sensitivity with respect to temperature and relative humidity is performed. Finally, a viscoelastic-based compensation technique is applied on the proposed wearable sensor not only to reduce its hysteresis and errors, but also to increase the sensor linearity. The sensor is validated on flexion and extension cycles with different angular velocities. Results show that the proposed sensor presents root mean squared errors of about 1.5° and mean hysteresis of about 1%. The wearable POF curvature sensor was applied on the angle measurement of an elbow joint during flexion and extension cycles and on the knee during the gait cycle, where high repeatability and low errors also were found.