Monogenic lupus offers valuable insights into the underlying mechanisms and therapeutic approaches for systemic lupus erythematosus (SLE). Here we report on five patients with SLE carrying recessive mutations in phospholipase D family member 4 (PLD4). Deleterious variants in PLD4 resulted in impaired single-stranded nucleic acid exonuclease activity in in vitro and ex vivo assays. PLD4 loss-of-function mutations led to excessive activation of Toll-like receptor 7 (TLR7) and TLR9. Downstream inflammatory signalling pathways, especially type I interferon signalling, were hyperactivated in patient dendritic cells. Pld4-deficient mice presented with autoimmunity and cell-intrinsic expansion of plasmacytoid dendritic cells and plasma cells. Pld4-deficient mice responded to the JAK inhibitor baricitinib, suggesting that targeting type I interferon may be a potential therapy for patients with PLD4 deficiency.