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Rapid advances in functional electronics bring tremendous demands on innovation toward effective designs of device structures. Yarn supercapacitors (SCs) show advantages of flexibility, knittability, and small size, and can be integrated into various electronic devices with low cost and high efficiency for energy storage. In this work, functionalized stainless steel yarns are developed to support active materials of positive and negative electrodes, which not only enhance capacitance of both electrodes but can also be designed into stretchable configurations. The as‐made asymmetric yarn SCs show a high energy density of 0.0487 mWh cm⁻² (10.19 mWh cm⁻³) at a power density of 0.553 mW cm⁻² (129.1 mW cm⁻³) and a specific capacitance of 127.2 mF cm⁻² under an operating voltage window of 1.7 V. The fabricated SC is then made into a stretchable configuration by a prestraining‐then‐releasing approach …