An electrothermochromic paper display composed of colorimetrically reversible polydiacetylenes (PDAs), utilizing screen printing on photopaper and patterning of electrically conductive wires on the reverse side, was developed. Heat generated by passing a current through the wires on the back of the photopaper induced a blue-to-red color transition of the PDA on the front at regions corresponding to the wires. This resulted in the generation of red-colored images, which disappeared when the supplied electric current was removed. The voltage at which the PDAs changed color could be controlled by using structured diacetylene (DA) monomers. A PDA-based seven-segment display has also been developed. By applying voltages, red-colored numeric digits from 0 to 9 can be displayed on the surface of the PDA layer.