This paper describes a two-degrees-of-freedom haptic interface to investigate the brain mechanisms of human motor control, which is capable of safely and gently interacting with human arm motion during functional magnetic resonance imaging (fMRI). A hydrostatic transmission separates the interface into a master and an MR-compatible slave system, allowing the placement of all interfering components outside the electromagnetic shield of the MR room.The transmission mirrors force and motion of the master actuators on the slave system placed close to the MR scanner. The parallel architecture takes advantage of the linear MR-compatible actuators and allows human subjects to perform reaching movements comfortably in the small workspace limited by thedimensions of the MR scanner and the biomechanics of the arm. The kinematic structure of the slave interface was optimized with respect to the available space and types of movements to beinvestigated. Materials were chosen based on their MR compatibility, their stiffness and weight. The interaction force with the subject is measured over two optical force sensors, located close to the output of the interface. Two shielded optoelectronic encoders measure the extension of the slave hydraulic pistons. Detailed tests demonstrated the fMRI compatibility even during movement of the interface.