The voltage-gated proton channel Hv1 regulates proton fluxes across membranes, thereby influencing pH-dependent processes. Plasmacytoid dendritic cells (pDCs) require a particularly tight regulation of endosomal pH to ensure strong type I IFN secretion exclusively during infection, avoiding autoimmunity. However, whether Hv1 is important for pH control in pDCs is presently unknown. In this study, we show that mouse pDCs require Hv1 to achieve potent type I IFN responses after the recognition of foreign DNA by endosomal TLR9. Genetic disruption of Hvcn1, which encodes Hv1, impaired mouse pDC activation by CpG oligonucleotides in vitro and in vivo, reducing IFN-α secretion and the induction of IFN-stimulated genes. Mechanistically, Hvcn1 deficiency delayed endosomal acidification and enhanced intracellular reactive oxygen species production, consequently limiting protease activity and TLR9 signaling. Our study reveals a critical role of Hv1 during innate immune responses and places this channel as a key modulator of type I IFN production, the hallmark function of pDCs, commending Hv1 as an attractive target for modulating type I IFN–driven autoimmunity.