Abstract: Thermoelectric fibers are flexible materials that can directly convert heat into electricity. They combine a high aspect ratio, excellent flexibility, and good weavability. These advantages give them strong potential in low-grade waste heat recovery, wearable electronics, and fire safety protection. Compared with conventional bulk and thin-film thermoelectric materials, thermoelectric fibers show clear advantages. They can conform to complex curved surfaces and tolerate repeated mechanical deformation. They are also suitable for large-scale and continuous processing. This review systematically summarizes recent progress in the design, fabrication, and application of thermoelectric fibers. First, the fabrication of fiber-based thermoelectric devices and their application advantages are briefly introduced. Then, electronic and ionic thermoelectric fibers are discussed in detail. Their material systems, performance characteristics, and corresponding fabrication methods are reviewed. Typical representative studies are highlighted. Furthermore, recent advances in applications are summarized, including wearable energy harvesting, environmental waste heat recovery, and intelligent fire safety systems. Finally, the main challenges faced by thermoelectric fibers are analyzed. These challenges include thermoelectric performance, mechanical stability, environmental durability, and scalable fabrication. Future perspectives are also discussed, focusing on material design, structural regulation, and multifunctional integration. This review provides useful strategies and references for the fabrication and application of thermoelectric fiber materials.