Flexible organic bioelectronic devices, which extract electronic signals from living systems, have been developed for sensing, recording, and monitoring various physiological states of biological systems. Organic electrochemical transistors (OECTs) have emerged as a promising platform for bioelectronics because of their inherent amplification function, high sensitivity, low cost, easy operation, and compatibility with flexible and wearable devices. This review provides a comprehensive overview of recent advancements in flexible OECTs for biosensing applications, including the fundamental principles and mechanisms of flexible OECTs, various channel materials used for biosensing, functionalization of OECTs for biosensing, use of flexible OECTs for the acquisition of biological signals, bioinformatics analysis of OECT-based biosensors, and development of biomimetic devices. The review concludes with a summary of the state-of-the-art technology for flexible OECT-based biosensors and the future outlook for this rapidly evolving field.