Flat mirrors, also known as flat parabolic surfaces, for millimeter-wave and terahertz imaging systems are demonstrated. This flat mirror is based on the metasurface in which an inexpensive printed circuit board technology is used to realize copper patterns printed on an FR4 substrate. Compared to the conventional reflector antennas used today in diverse applications (for homeland security, medical systems, communication, etc.), the suggested mirror has major advantages in process simplicity, mechanical flexibility, frequency alignment, weight, and cost. The theoretical background, simulation results, experimental results, and proof of concept are given in this Letter.