Organizing nanoparticles (NPs) into periodic structures is a central goal in materials science. Despite progress in the last decades, it is still challenging to produce macroscopic assemblies reliably. In this work, we report the analysis of the pervaporation-induced organization of gold octahedra into supercrystals within microfluidic channels using a combination of X-ray scattering techniques and FIB-SEM tomography. The results reveal the formation of a singledomain supercrystal with a monoclinic C2/m symmetry and long-range order extending over the dimensions of the microfluidic channel, covering at least 1.7x0.3 mm 2. Time-resolved small angle X-ray scattering analysis showed that the formation of the superlattice involves an accumulation of the NPs within the channel before the nucleation and growth of the supercrystal. The orientation of the crystal remains unchanged during its formation, suggesting a growth mechanism directed by the channel interface. Together, these results show the potential application of the pervaporation strategy to providing spatially determined control over NP crystallization, which can be used for the rational fabrication of nanomaterial architectures.