The excessive production of type I IFNs is a hallmark and a main pathogenic mechanism of many autoimmune diseases, including systemic lupus erythematosus (SLE). In these pathologies, the sustained secretion of type I IFNs is dependent on the improper activation of plasmacytoid DCs (pDCs) by self-nucleic acids. However, the nature and origin of pDC-activating self-nucleic acids is still incompletely characterized. Here, we report that exosomes isolated from the plasma of SLE patients can activate the secretion of IFN-α by human blood pDCs in vitro. This activation requires endosomal acidification and is recapitulated by microRNAs isolated from exosomes, suggesting that exosome-delivered microRNAs act as self-ligands of innate single-stranded endosomal RNA sensors. By using synthetic microRNAs, we identified an IFN induction motif that is responsible for the TLR7-dependent activation, maturation, and survival of human pDCs. These findings identify exosome-delivered microRNAs as potentially novel TLR7 endogenous ligands able to induce pDC activation in SLE patients. Therefore, microRNAs may represent novel pathogenic mediators in the onset of autoimmune reactions and potential therapeutic targets in the treatment of type I IFN-mediated diseases.