Low-dimensional materials have recently attracted much interest as thermoelectric materials because of their charge carrier confinement leading to thermoelectric performance enhancement. Carbon nanotubes are promising candidates because of their one-dimensionality in addition to their unique advantages such as flexibility and light weight. However, preserving the large power factor of individual carbon nanotubes in macroscopic assemblies has been challenging, primarily due to poor sample morphology and a lack of proper Fermi energy tuning. Here, we report an unprecedentedly high value of power factor (14±5 mWm-1K-2) for centimeter-long weavable fibers of aligned carbon nanotubes with ultrahigh electrical and thermal conductivity. Our theoretical simulations show that the observed giant power factor originates from the one-dimensional quantum confinement of charge carriers, appearing when the Fermi energy is near a van Hove singularity in the electronic density of states. We fabricated a textile thermoelectric generator based on these carbon nanotube fibers, which demonstrated high thermoelectric performance, weavablity, and scalability. The giant power factor we observed make these fibers strong candidates for the emerging field of thermoelectric active cooling, which requires a large thermoelectric power factor and a large thermal conductivity at the same time.