After decades of theoretical study in physiology and neurology communities, the paradigm of muscle synergies is now being explored in rehabilitation robotics as a strategy to control mechanical artifacts with many degrees-of-freedom (DoF) in a simple yet effective and human-like way. In particular, muscle synergies during grasping and in graded-force tasks are of great interest for the control of dexterous hand prostheses. To this end, we have designed and tested a novel device to accurately and simultaneously measure fingertip forces. The device, called FFLS (Finger-Force Linear Sensor), measures the forces applied by the human fingertips in both directions (flexion and extension of index, middle, ring and little finger plus thumb rotation and abduction/adduction). It is suited for several different hand sizes, enforces high accuracy in the measurement and its signal is guaranteed to be linear in a high range of forces (100N in both directions for each finger). It outputs six analog voltages (±10V), suited for processing with a DAQ card.