Assembly of a eukaryotic nucleus involves three distinct events: membrane recruitment, fusion to form a double nuclear membrane, and nuclear pore complex (NPC) assembly. We report that importin β negatively regulates two of these events, membrane fusion and NPC assembly. When excess importin β is added to a full Xenopus nuclear reconstitution reaction, vesicles are recruited to chromatin but their fusion is blocked. The importin β down-regulation of membrane fusion is Ran-GTP reversible. Indeed, excess RanGTP (RanQ69L) alone stimulates excessive membrane fusion, leading to intranuclear membrane tubules and cytoplasmic annulate lamellae-like structures. We propose that a precise balance of importin β to Ran is required to create a correct double nuclear membrane and simultaneously to repress undesirable fusion events. Interestingly, truncated importin β 45-462 allows membrane fusion but produces nuclei lacking any NPCs. This reveals distinct importin β-regulation of NPC assembly. Excess full-length importin β and β 45-462 act similarly when added to prefused nuclear intermediates, i.e., both block NPC assembly. The importin β NPC block, which maps downstream of GTPγS and BAPTA-sensitive steps in NPC assembly, is reversible by cytosol. Remarkably, it is not reversible by 25 μM RanGTP, a concentration that easily reverses fusion inhibition. This report, using a full reconstitution system and natural chromatin substrates, significantly expands the repertoire of importin β. Its roles now encompass negative regulation of two of the major events of nuclear assembly: membrane fusion and NPC assembly.