Abstract: Yarn-based flexible sensors, owing to their distinctive flexibility, textile processability, and capability for multifunctional integration, have emerged as key technological components for enabling adaptive sensing, human–machine interaction, and sustainable power supply in smart textiles. Compared with conventional rigid or film-based wearable devices, yarn-based sensors can be more readily incorporated into textile structures while maintaining wearing comfort and mechanical compliance. However, the matching relationships among different sensing mechanisms, textile integration strategies, and application scenarios, as well as the synergistic balance among sensing performance, durability, and wearing comfort, have not yet been systematically clarified. Focusing on the integration and application of yarn-based flexible sensors in smart textiles, this review summarizes recent progress in yarn-based flexible mechanical, temperature, and humidity sensors, together with their representative fabrication methods. In addition, the roles of three typical textile integration techniques, namely sewing, embroidery, and weaving, are discussed with emphasis on their effects on improving the functional coordination between sensing units and textile substrates. From the perspective of smart textile applications, yarn-based sensors have enabled multidimensional and real-time monitoring of human physiological signals, motion postures, and environmental parameters. Finally, the key technical challenges currently facing this field are analyzed, and potential future development directions are proposed to provide guidance for the design and practical application of next-generation intelligent textile systems.