Flexible fiber‐shaped sodium dual‐ion batteries (FSDIBS) as a proof of concept are fabricated by using the hierarchical ReS2 nanosheets anchored on the carbon nanotube (ReS2@CNT) fiber as anode and graphite on the CNT as cathode. Owing to large interlayer spacing and weak layer coupling force of the ReS2 nanosheets and the anion accommodation of the graphite combined with good flexibility of the CNT fiber, the FSDIBS demonstrate outstanding electrochemical performances with high working voltage and high specific volumetric energy density, durable cycling life, and good flexibility. The FSDIBS show a specific discharge capacity of 97.8 mAh cm−3 at a current density of 630 mA cm−3 and high specific energy density of 25.12 mWh cm−3 (based on the whole volume of the two electrodes) and superb stability with a capacity retention of 91.8% even after bending for 2100 cycles. Moreover, a series of ex situ/in situ characterizations are verified that the reversible shuttles of the Na+ cations and PF6‐ anions between the anode and cathode are simultaneously occurred during the charge/discharge process. A novel fiber‐shaped sodium dual‐ion battery (FSDIB) is fabricated, which reveals excellent electrochemical properties, superb flexibility, and high energy density.