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• Stretchable gas sensors were developed based on microstructured MXene/PU core-sheath fibers. • The sensor exhibited higher sensitivity and SNR than that of IE-based planar sensors. • Sensitivity and stretchability can be tailored through microcracks/wrinkles engineering. • Workability at both relaxed and stretching states was realized with minimal strain interference. Emerging wearable electronics pose urgent needs for high-performance, wearable gas sensors. However, the state-of-the-art gas sensors based on planer interdigited electrodes (IE) are commonly flexible yet not stretchable, which is not desirable for integration with human skin or clothing. Here, we report the fabrication of a stretchable and wearable gas sensor by using MXene/polyurethane (PU) core-sheath fibers. The synergetic effects of charge transfer and swelling-induced stretching offer the MXene/PU fiber sensor with high sensitivity to acetone (5–325 fold higher than planer MXene sensor deposited on an IE), wide sensing range (from ppb level to saturated vapor), and high signal-to-noise ratio (SNR, 160 % higher than planer MXene sensor). To further improve the sensing performance of the MXene/PU fiber, microstructures including microcracks and microwrinkles were developed into the fiber sheath. The microcracks can amplify the swelling-induced resistance variation of the conductive sheath; and therefore, leading to an enhanced sensing response 40 % higher than the flat core-sheath fiber. Furthermore, by developing microwrinkles onto the conductive sheath, a highly stretchable fiber-based gas sensor has been successfully demonstrated, which can accommodate human skin deformation (30 %) with a minimal sensing interference from tensile strain. The as-reported MXene/PU fibers can be knitted into clothing and serve as wearable gas sensors with good sensing reliability and gas permeability. Our results provide insight in utilizing microstructure engineering to tailor the sensing performance of gas sensors. [ABSTRACT FROM AUTHOR]