<p>Rising textile fiber demand over the past two decades has intensified interest in the potential of wood-based fibers and recycled fibers to substitute for conventional cotton and synthetic fibers. However, the sector-level substitution impacts remain inadequately explored, especially regarding the diverse substitution patterns—among wood-based fibers, among non-wood fibers, and between wood- and non-wood fibers—and how these evolve over time. By integrating non-linear fiber demand trajectories, empirical substitution ratios, and the effect of decarbonization, we estimated annual substitution impacts and net emissions for the global textile fiber sector from 2022 to 2050. The results show that at the sector level, emission-causing substitutions among non-wood fibers—primarily polyester replacing cotton—outweigh the emission-avoiding substitutions, including wood-based fibers replacing cotton and recycled polyester displacing virgin polyester, leading to an increase in emissions under a business-as-usual demand trajectory. In contrast, increasing the shares of novel wood-based fibers and recycled polyester shifts the aggregate substitution impact from emission-causing to emission-avoiding, regardless of whether demand growth accelerates or slows. However, these gains from substitution are outweighed by emissions from marginal new demand that do not displace other fibers due to imperfect substitution, leading to net emissions. Nevertheless, reduced demand growth narrows the gap between the overall emissions avoided through substitution and the emissions induced by marginal new demand. These results indicate that a climate-aligned textile transition cannot rely on substitution and recycling alone, but must also involve moderating demand growth, increasing recycling rates and extending product lifetimes.</p>