Nanoneedles are emerging as a safe and scalable strategy for the genetic modification of primary human cells. However, a limited understanding of how interactions at the biointerface lead to functional gene expression continues to hinder clinical translation. While direct membrane penetration, permeabilization, and endocytosis have been proposed as intracellular delivery avenues, the mechanistic connection between delivery and successful transfection remains unclear. Here, we identify caveolae-mediated endocytosis, dependent on Caveolin-1, as a key mechanism enabling nanoneedle transfection. By selectively modulating Caveolin-1 expression in primary human regulatory T cells and MG63 cells and investigating endolysosomal processing, we show that although nucleic acids can be efficiently delivered in the absence of Caveolin-1, gene expression occurs only when caveolar endocytosis is present. These findings reveal a mechanistic basis and establish a broader design principle for nanoneedle transfection: interfacing must be accompanied by the engagement of permissive cellular trafficking pathways to achieve gene expression.