—A robotic device, aimed at assisting a urologist who positions an endorectal ultrasound probe to perform prostate biopsies, is presented. The proposed system is a comanipulator that holds the probe simultaneously with the urologist. This robot supports two modes of operation: the free mode, where the entire movement control is left to the urologist when he/she positions the probe with respect to the prostate thanks to the feedback provided by the ultrasound images; the locked mode, where the robot's role is to precisely maintain the targeted biopsy site at a given location, while the urologist can insert a needle through a guide mounted to the probe and proceed to biopsy. The device combines three brakes and three motors. This allows both transparent comanipulation in the free mode with 6 Degrees of Freedom (DoFs) liberated and stabilization of the probe in the locked mode. At the control level, a main challenge in the locked mode raises from antagonistic constraints: the needle placement shall be precise in spite of unknown external forces due to the contact between the probe and the rectum; the robot apparent impedance shall be low due to security constraints. This is solved by an inner low stiffness controller and an outer slow integration for canceling steady state errors. Both in vitro and in cadavero experimental results show the efficiency of the system in the two modes of operation.