This paper describes the development of a three-dimensional (3D) printing system that integrates a six-degree-of-freedom industrial robot into a fused deposition modeling process. By using the robot-based 3D printing system, printing on inclined planes became possible, which cannot be achieved by a conventional 3D printer. Moreover, the robotic 3D printing is supposed to achieve faster and smoother motion compared to its counterpart under the same temporal settings, thanks to a knowledge-based strategy to re-plan printing trajectories from a set of G-commands. The accurate execution of the printing trajectories and other necessary components for the printing process (for example, an extruder) are regulated by the robot operating system (ROS). The efficiency of the printing system was evaluated by 3D printing a couple of simple 3D models using a six-axis Denso robot. The preliminary results revealed great potential for rapid prototyping and printing in close contact with humans, especially in the field of interactive manufacturing, or human-robot collaboration.