Flexible electronics is now one of key technologies for electronic devices that collect information from nonplanar surfaces, ultimately enabling the realization of the trillion sensor concept and the internet of things. Both flexible discrete devices and sensors are required for integration of digital and analog circuits for signal processing. In this study, flexible, energy efficient, complementary metal‐oxide‐semiconductor (CMOS) digital and analog circuits, monolithically integrated with a temperature sensor are reported: This is achieved by developing the process of p‐type and n‐type transistors on a flexible substrate. For the digital circuits, CMOS‐based J–K flip‐flop circuits are fabricated on a polyimide substrate, and testing confirms that they can be operated under mechanical bending without malfunction. Furthermore, the fabrication of a flexible analog differential amplifier is demonstrated, with the device successfully amplifying the output of an integrated temperature sensor by ≈38 times. Although these demonstrations are still at the proof‐of‐concept stage for flexible signal processing circuits, these achievements are a key step toward the next level of device integration, thereby enabling the development of viable flexible electronic devices.