Despite the importance of carrier mobility, recent research efforts have been mainly focused on the improvement of volumetric capacitance in order to maximize the figure‐of‐merit, μC* (product of carrier mobility and volumetric capacitance), for high‐performance organic electrochemical transistors. Herein, high‐performance microfiber‐based organic electrochemical transistors with unprecedentedly large μC* using highly ordered crystalline poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) microfibers with very high carrier mobilities are reported. The strain engineering via uniaxial tension is employed in combination with solvent‐mediated crystallization in the course of drying coagulated fibers, resulting in the permanent preferential alignment of crystalline PEDOT:PSS domains along the fiber direction, which is verified by atomic force microscopy and transmission wide‐angle X‐ray scattering. The resultant strain‐engineered microfibers exhibit very high carrier mobility (12.9 cm2 V−1 s−1) without the trade‐off in volumetric capacitance (122 F cm−3) and hole density (5.8 × 1020 cm−3). Such advantageous electrical and electrochemical characteristics offer the benchmark parameter of μC* over ≈1500 F cm−1 V−1 s−1, which is the highest metric ever reported in the literature and can be beneficial for realizing a new class of substrate‐free fibrillar and/or textile bioelectronics in the configuration of electrochemical transistors and/or electrochemical ion pumps.

To realize thermoelectric textiles that can convert body heat to electricity, fibers with excellent mechanical and thermoelectric properties are needed. Although poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is among the most promising organic thermoelectric materials, reports that explore its use for thermoelectric fibers are all but absent. Herein, the mechanical and thermoelectric properties of wet‐spun PEDOT:PSS fibers are reported, and their use in energy‐harvesting textiles is discussed. Wet‐spinning into sulfuric acid results in water‐stable semicrystalline fibers with a Young's modulus of up to 1.9 GPa, an electrical conductivity of 830 S cm−1, and a thermoelectric power factor of 30 μV m−1 K−2. Stretching beyond the yield point as well as repeated tensile deformation and bending leave the electrical properties of these fibers almost unaffected. The mechanical robustness/durability and excellent underwater stability of semicrystalline PEDOT:PSS fibers, combined with a promising thermoelectric performance, opens up their use in practical energy‐harvesting textiles, as illustrated by an embroidered thermoelectric fabric module.