Nuclear envelope (NE) reassembly is essential for the zygote-to-totipotency transition in mammalian embryos. Beyond its classical roles in nuclear transport and maintenance of the RanGTP gradient, the function of RanGAP1 in embryonic NE reassembly has remained elusive. Here, we show that RanGAP1 localizes to the early embryonic NE, driving reassembly via a distinct structural mechanism. Trim-Away-mediated RanGAP1 depletion causes two-cell stage arrest and severe NE defects, including nuclear pore complex (NPC) loss, failed reassembly, and aberrant "fishnet" membrane structures. Mechanistically, RanGAP1 establishes a structural paradigm for endoplasmic reticulum (ER)-lamina coupling by recruiting Lamin A/C, B1, and B2 to the ER for coassembly. The acidic domain and SUMO-1 modification at Lys526 (K526) are essential for the NPC localization of RanGAP1. Thus, RanGAP1 orchestrates nuclear lamina (NL) assembly by targeting lamins to the ER. These findings uncover mechanisms of NE reassembly that are critical for early embryogenesis and provide insights into NE-related developmental disorders.