In this paper, the synthesis of ultrasmall Na₂FePO₄F nanoparticles (≈3.8 nm) delicately embedded in porous N‐doped carbon nanofibers (denoted as Na₂FePO₄F@C) by electrospinning is reported. The as‐prepared Na₂FePO₄F@C fiber film tightly adherent on aluminum foil features great flexibility and is directly used as binder‐free cathode for sodium‐ion batteries, exhibiting admirable electrochemical performance with high reversible capacity (117.8 mAh g⁻¹ at 0.1 C), outstanding rate capability (46.4 mAh g⁻¹ at 20 C), and unprecedentedly high cyclic stability (85% capacity retention after 2000 cycles). The reaction kinetics and mechanism are explored by a combination study of cyclic voltammetry, ex situ structure/valence analyses, and first‐principles computations, revealing the highly reversible phase transformation of Na₂Fe^IIPO₄F ↔ NaFe^IIIPO₄F, the facilitated Na⁺ diffusion dynamics with low energy barriers, and the desirable pseudocapacitive behavior for fast charge storage. Pouch‐type Na‐ion full batteries are also assembled employing the Na₂FePO₄F@C nanofibers cathode and the carbon nanofibers anode, demonstrating a promising energy density of 135.8 Wh kg⁻¹ and a high capacity retention of 84.5% over 200 cycles. The distinctive network architecture of ultrafine active materials encapsulated into interlinked carbon nanofibers offers an ideal platform for enhancing the electrochemical reactivity, electronic/ionic transmittability, and structural stability of Na‐storage electrodes.