AbstractWith the development of phase engineering of nanomaterials (PEN), construction of noble‐metal heterostructures with unconventional crystal phases, including heterophases, has been proposed as an attractive approach toward the rational design of highly efficient catalysts. However, it still remains challenging to realize the controlled preparation of such unconventional‐phase noble‐metal heterostructures and explore their crystal‐phase‐dependent applications. Here, various Pd@Ir core–shell nanostructures are synthesized with unconventional fcc‐2H‐fcc heterophase (2H: hexagonal close‐packed; fcc: face‐centered cubic) through a wet‐chemical seeded method. As a result, heterophase Pd₆₆@Ir₃₄ nanoparticles, Pd₄₅@Ir₅₅ multibranched nanodendrites, and Pd₆₈@Ir₂₂Co₁₀ trimetallic nanoparticles are obtained via the phase‐selective epitaxial growth of fcc‐2H‐fcc‐heterophase Ir‐based nanostructures on 2H‐Pd seeds. Importantly, the heterophase Pd₄₅@Ir₅₅ nanodendrites exhibit excellent catalytic performance toward electrochemical hydrogen evolution reaction (HER) under acidic conditions. An overpotential of only 11.0 mV is required to achieve a current density of 10 mA cm⁻² on Pd₄₅@Ir₅₅ nanodendrites, which is lower than those of the conventional fcc‐Pd₄₇@Ir₅₃ counterparts, commercial Ir/C and Pt/C. This work not only demonstrates an appealing route to synthesize novel heterophase nanomaterials for promising applications in the emerging field of PEN, but also highlights the significant role of the crystal phase in determining their catalytic properties.