<p>The equilibrium shape and surface termination of magnetite nanoparticles (NPs) are fundamental properties that determine the physical and chemical properties of the supercrystal structure of oleic acid-coated magnetite NPs. Here, we studied the equilibrium shape of magnetite (Fe₃O₄) NPs supported by Al₂O₃(0001) single-crystalline surfaces. We report the growth of epitaxial (111)-oriented NPs exhibiting a triangular shape with a height-to-diameter aspect ratio of 0.42 over a wide growth temperature range. We probed the surface termination of the NP facets by adsorbing formic acid as a prototypical molecule representing the adsorption behavior of oleic acid. We identified infrared absorption bands characteristic of dissociative adsorption on (111) facets with the iron tetrahedral (Fe-tet₁) termination, as well as on mixed-terminated (100) side facets. Our experimental findings are supported by predictions of the NP shape using surface-free energy calculations from ab initio thermodynamics. The experimentally observed nanoparticle shape can only be rationalized by the presence of bulk-terminated {100} type facets. Such a fundamental understanding of the shape and surface terminations of oxide nanoparticles is crucial for tailoring the properties of hybrid hierarchical materials and drug carriers and for their development.</p>