This paper proposes a force/torque sensor structure that can be easily integrated with a flexible manipulator structure. The sensor’s ring-like structure with its hollow inner section provides ample space for auxiliary components, such as cables and tubes, to be passed through and, hence, is very suitable for integration with tendon-driven and fluid-actuated manipulators. The sensor structure can also accommodate the wiring for a distributed sensor system as well as for diagnostic instruments that may be incorporated in the manipulator. Employing a sensing approach based on optical fibers as done here allows for the creation of sensors that are free of electrical currents at the point of sensing and immune to magnetic fields. These sensors are inherently safe when used in the close vicinity of humans and their measuring performance is not impaired when they are operated in or nearby machines such as magnetic resonance imaging (MRI) scanners. This type of sensor concept is particularly suitable for inclusion in instruments and robotic tools for minimally invasive surgery (MIS). The paper summarizes the design, integration challenges and calibration of the proposed optical three-axis force sensor. The experimental results confirm the effectiveness of our optical sensing approach and show that after calibrating its stiffness matrix, force and momentum components can be determined accurately.