Thick, flexible electrodes are essential to simultaneously achieving flexibility and high energy density; however, mechanical failure and the sluggish movement of ions and electrons both restrict their application. Here, a thick electrode reinforced by a stainless-steel (SS) fiber three-dimensional (3D) current collector is proposed that simultaneously attains unprecedented flexibility and a high energy density. This ultra-flexible electrode is prepared by a thermally induced phase separation process. Its meso/macroporosity enhances ionic conductivity, and the 3D fiber reinforcement enhances interfacial adhesion, flexural durability, and electrical conductivity. Owing to these advantages, the fiber-reinforced electrode has a minimum bending radius of 3 mm owing to its high yield strain (13%) and attains a high energy density of 500 Wh·L-1, which is considerably higher than that of previous flexible batteries (100–350 Wh·L-1). In contrast with the same electrode coated on metal foil, which suffers from delamination, the fiber-reinforced electrode is delamination-free and outperforms in rate capability and cycling performance. Unlike conventional current collectors (foil, mesh, or foam), the SS fiber can be tailored to be distributed throughout the electrode and to fit the electrode form factor. Fiber-reinforced electrodes are also excellent at creating 3D free-form batteries, which are difficult to fabricate with conventional electrode structures.