Chemical and biological sensing play important roles in healthcare, environmental science, food-safety tests, and medical applications. Flexible organic electrochemical transistors (OECTs) have shown great promise in the field of chemical and biological sensing, owing to their superior sensitivity, high biocompatibility, low cost, and light weight. Herein, we summarize recent progress in the fabrication of flexible OECTs and their applications in chemical and biological sensing. We start with a brief introduction to the working principle, configuration, and sensing mechanism of the flexible OECT-based sensors. Then, we focus on the fabrication of flexible OECT-based sensors, including material selection and structural engineering of the components in OECTs: the substrate, electrodes, electrolyte, and channel. Particularly, the gate modification is discussed extensively. Then, the applications of OECT-based sensors in chemical and biological sensing are reviewed in detail. Especially, the array-based and integrated OECT sensors are also introduced. Finally, we present the conclusions and remaining challenges in the field of flexible OECT-based sensing. Our timely review will deepen the understanding of the flexible OECT-based sensors and promote the further development of the fast-growing field of flexible sensing.