Shape and size of nanoparticles are fundamental structural properties that govern the development of novel surface-related applications. Traditionally external agents such as surfactants, reducing agents, or stabilizers have been used for enforcing preferential growth orientation, size, and shape to develop tailor-made nanoparticles. However, these external agents cover the pristine surface of the particle and invariably reduce the surface activity. Here, we introduce a surfactant-free, single-step electrochemical method to control the shape of air-stable FeNi alloy nanoparticles. Using glancing-incidence X-ray diffraction, we further demonstrate that the shape evolution of nanoparticles from concave cube to truncated sphere occurs concurrently with the phase transformation from bcc to fcc. This shape evolution can be achieved by fine-tuning a single parameter, the ratio of reactant concentrations (i.e., [Ni2+]/[Fe2+]). Addition of Ni2+ to the Fe2+ electrolyte changes the nucleation mechanism from progressive growth for pure Fe2+ electrolyte to instantaneous growth for mixed Fe2+/Ni2+ electrolyte, which leads to a remarkably narrow size distribution and very uniform dispersion on the Si substrate. Depth-profiling X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis by both transmission electron microscopy and scanning electron microscopy for nanoparticles at different growth stages reveal alloy formation and preferential deposition of Fe during initial growth that results in a quasi-core–shell structure. We also observe the in-situ formation of a very thin Ni-doped FeOOH outer layer and NiFe2O4 intermediate layer on the skin of the nanoparticles, which passivates the surface and dramatically enhances the air stability. The present work provides a unique example of shape-controlled bimetallic nanostructures and offers insights into growth modification of a host metal structure by a guest metal.