We describe a non-conventional, MOF-based approach with modified linkers to fabricate 3D Bi2O3 supracrystals. The nanoparticle (NP) assembly exhibits bcc-packing, which is difficult to achieve with other methods. The NPs possess a very narrow size distribution. The individual NPs were synthesized inside the pores of a surface-mounted metal-organic framework (SURMOF) template via a photo-decomposition procedure. The supracrystals were thoroughly characterized using X-ray diffraction (XRD), infrared (IR) and Raman spectroscopy as well as high-resolution transmission electron microscopy (HR-TEM) and SAED (Selected Area Electron Diffraction). In order to achieve sharp size distributions of the NPs, the pores within the SURMOF were functionalized with amino (-NH2) functional groups acting as nucleation centers. MOFs lacking such additional functionalities, Cu3(BTC)2, yielded much broader size distributions. These findings provide a unique molecular design tool for creating nanometer-sized reaction compartments for the synthesis of supracrystals with packing types not accessible via self-assembly.