Flexible and stretchable biosensors can offer seamless and conformable biological-electronic interfaces for continuously acquiring high-fidelity signals, permitting numerous emerging applications. Organic thin film transistors (OTFTs) are ideal transducers for flexible and stretchable biosensing due to their soft nature, inherent amplification function, biocompatibility, ease of functionalization, low cost, and device diversity. In consideration of the rapid advances in flexible OTFT-based biosensors and their broad applications, herein, we provide a timely and comprehensive review. We start with a detailed introduction to the features of various OTFTs including organic field-effect transistors (OFETs) and organic electrochemical transistors (OECTs), and the functionalization strategies for biosensing, with a highlight on the seminal work and up-to-date achievements. Then, we detail the applications of flexible OTFT-based biosensors in wearable, implantable, and portable electronics, as well as neuromorphic biointerfaces. Subsequently, special attention is paid to emerging stretchable organic transistors including planar and fibrous devices. The routes to impart stretchability, including structural engineering and material engineering, are discussed, and the implementations of stretchable organic transistors in e-skin and smart textiles are included. Finally, we summarize the remaining challenges and the future opportunities in this field. This article is protected by copyright. All rights reserved.