Grain-boundary engineering is pivotal to fully utilize the mechanical, electrical, and thermal-transport properties of various materials. However, current methods in metallurgy rely almost exclusively on top-down approaches, making precise grain-boundary engineering, especially at nanoscale, difficult to achieve. Herein, we report a method to produce tailored grain-boundary conditions with nanoscale precision from colloidal metal nanocrystals through surface treatment followed by a pressure-sintering process. The resulting bulk grain-boundary materials (which we call “nanocrystal coins”) possess a metal-like appearance and conductivity while inheriting the original domain features of the nanocrystal building blocks. Nanoindentation measurements confirmed the superior mechanical hardness of the obtained materials. Further, we use this method to fabricate, for the first time, a single-component bulk metallic glass from amorphous palladium nanoparticles. Our discovery may spur the development of new materials whose functionality crucially depends on the domain configuration at nanoscale, such as superhard materials, thermoelectric generators, and functional electrodes. Display omitted •Fabrication of bulk grain-boundary materials from eight kinds of metal nanoparticles•Precise nanoscale grain-boundary engineering using nanocrystals•Emerging the Hall-Petch effect to enhance mechanical hardness of the materials•The first example of a single-component bulk metallic glass Enhancing materials’ properties through grain-boundary (GB) engineering has been broadly employed in various scenes, ranging from an ancient “striking while the iron is hot” approach to modern state-of-the-art techniques. However, precisely controlling the GB condition of bulk materials at the nanometer scale has proven to be extremely challenging. In this work, we discover a new GB engineering approach, i.e., a nanocrystal (NC)-coining process, through consolidating premade NCs into bulk GB materials under pressure. By using surface-engineered metal NCs as the building blocks of “nanograins,” free-standing metal pieces (which we call “NC coins”) with designed nanometer-sized domains can be produced. These NC coins show metallic appearance, conductivity, and enhanced mechanical hardness. Our method allows us to create the first single-component bulk metallic glass from amorphous palladium nanoparticles. We propose a new concept, the nanocrystal (NC)-coining process, to produce bulk materials with precisely tailored nanoscale grain-boundary conditions. By consolidating surface-engineered NCs into bulk materials using pressure, we can fabricate free-standing NC coins with metallic appearance and conductivity, while preserving the original NC domain feature. The obtained NC coins show enhanced mechanical hardness due to the Hall-Petch effect. With this method, we have created the first example of a bulk single-component metallic glass from amorphous palladium nanoparticles.