Surgical robots, including those with remote center of motion (RCM) mechanisms, have demonstrated utility in image-guided percutaneous needle placement procedures. However, widespread clinical application of these robots is hindered by not only complicated mechanical design but also the need for calibration and registration of the robot to the medical imager prior to each use. In response, we propose a Virtual RCM algorithm that requires only online tracking or registering the surgical tool to the imager, and a five degree-offreedom (DOF) robot comprised of three prismatic DOF and two rotational DOF. The robot can be unencoded, uncalibrated, and does not require preoperative registration. An incremental adaptive motion control cycle both guides the needle to the insertion point and orients it to align with the target. The robot executes RCM motion "virtually" without having a physically constrained fulcrum point. The proof-of-concept prototype system achieved 0.78 mm translation and 0.78 degrees rotational accuracy (within the tracker accuracy), within 17 iterative steps (0.5-1secs).