Multiferroic materials, characterized by the coexisting of ferroelectric polarization (breaking space‐ inversion symmetry, 𝒫) and magnetism (breaking time‐reversal symmetry, 𝒯), with strong magnetoelectric coupling, are highly sought after for advanced technological applications. Novel altermagnets, distinct from conventional magnets, have recently been revealed to exhibit unique spin polarization protected by crystal symmetry, which naturally overcomes the isolation of magnetism from ferroelectrics associated with spatial symmetry. In this study, a novel class of type‐III multiferroics is proposed, which leverages the unique symmetry of altermagnets to enforce spin‐ferroelectric locking, setting them apart from conventional multiferroics. Through first‐principles calculations, ferroelectric switching is shown to fully invert the spin polarization of altermagnets, equivalent to a 180° reversal of magnetic spin. This altermagnetic phase controlled by ferroelectrics can be effectively probed using the magneto‐optical Kerr effect, revealing a new class of multiferroics with intrinsic and deterministic magnetoelectric coupling. This theoretical advancement redefines the design principles of magnetoelectric materials and lays the foundation for the design of next‐generation spintronic devices leveraging altermagnetism.

With the significant advancement of portable/wearable electronics, the demand for flexible electronic devices has significantly increased; in the field of energy storage, the development of dielectric capacitors is still facing challenges due to the difficulty in integrating large recoverable energy storage density (W_rec), high efficiency (η), and robust bendability with fully flexible pulsed power systems operating in harsh environments. Herein, the all-inorganic flexible Mn-doped (1−x)(0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃)-xSrTiO₃ (Mn:NBT-BT-xST, x = 0.30, 0.45, 0.60, and 0.75) film capacitors were obtained via simple one-step fabrication on a mica substrate. The Mn:NBT-BT-0.45ST film capacitor shows the desirable W_rec of 76.1 J cm⁻³ and the high η of 80.0% owing to the predominant relaxor feature and weak antiferroelectric-like behavior. Furthermore, stable properties in the frequency range from 500 Hz to 20 kHz and the ultra-wide working temperature range of −100 to 200 °C and robust fatigue and retention endurance over 10⁸ cycles and 10³ s were achieved, respectively. Moreover, mechanical bending processes at the small radius of 2 mm or even 10⁴ mechanical bending cycles had no influence on the energy storage performances. This study is expected to pave the way for the application of high-performance bendable dielectric film capacitors in energy storage and flexible electronic devices.