conventional nanostructures, for instance with grain sizes below a critical value of ≈50 nm, can experience softening due to grain boundary sliding. [11,12] Over the past years, a novel approach to microstructure design, known as hierarchical laminate structure, has shown to greatly improve strength and ductility of conventional materials simultaneously, such as Ti alloys and steels, through massive microstructure refinement. [13][14][15][16] Typically, such materials with hierarchical laminate structures consist of a parent phase serving as soft matrix to improve ductility and a hard product phase impeding dislocation motion to strengthen the microstructure. Unfortunately, such heterogeneous materials with high local mechanical disparity among their constituting phases are often prone to internal damage initiation, thus limiting ductility to 3.8-15% for Ti alloys, [13,14] 20-33% for medium Mn steels, [17] and 23-33% for stainless steels. [16] Therefore, the mechanical properties of such hierarchical laminate materials need to be further improved to minimize the probability of internal damage and failure. Also, creating laminate structures in bulk materials often requires imposing complex processing methods, e.g., accumulative roll bonding. [18] Interestingly, a recently developed dual-phase high-entropy alloy (HEA) also exhibits a simultaneous increase of strength and ductility after grain refinement [19][20][21][22][23] as highlighted in Figure 1. The excellent strength-ductility combination of the novel dual-phase HEA has been reported to be related to the transformationinduced plasticity (TRIP) effect, i.e., the displacive phase transformation from the face-centered cubic (FCC γ) matrix into the hexagonal close-packed (HCP ε) phase upon deformation. [19,22] Depending on the magnitude of the stacking fault energy, different compositional variants of recently developed HEAs have shown deformation modes with planar dislocation slip, [24,25] twinning induced plasticity (TWIP), [26] or TRIP effect. [22] Here, we observe another state which is thermodynamically characterized by a similar stability of two co-existing phases (FCC γ and HCP ε). This means that the stacking fault energy of the FCC matrix must assume a near-zero yet positive value so that forward (γ → ε) and backward (ε → γ) transformations can be both triggered in the same material and loading state.We studied the deformation mechanisms in the dual-phase HEA (Fe 50 Mn 30 Co 10 Cr 10 , at%) in more detail via transmission electron microscopy (TEM) analysis and found that this novel TRIP-assisted HEA gradually develops a hierarchical Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, compromising the material's ductility. Here, a fundamentally new approach that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy allo...