SLAMF9 belongs to the conserved lymphocytic activation molecule family (SLAMF). Unlike other SLAMs， which have been extensively studied， the role of SLAMF9 in the immune system remained mostly unexplored. By generating CRISPR/Cas9 SLAMF9 knockout mice， we analyzed the role of this receptor in plasmacytoid dendritic cells (pDCs)， which preferentially express the SLAMF9 transcript and protein. These cells display a unique capacity to produce type I IFN and bridge between innate and adaptive immune response. Analysis of pDCs in SLAMF9 mice revealed an increase of immature pDCs in the bone marrow and enhanced accumulation of pDCs in the lymph nodes. In the periphery， SLAMF9 deficiency resulted in lower levels of the transcription factor SpiB， elevation of pDC survival， and attenuated IFN-α and TNF-α production. To define the role of SLAMF9 during inflammation， pDCs lacking SLAMF9 were followed during induced experimental autoimmune encephalomyelitis. SLAMF9 mice demonstrated attenuated disease and delayed onset， accompanied by a prominent increase of immature pDCs in the lymph node， with a reduced costimulatory potential and enhanced infiltration of pDCs into the central nervous system. These results suggest the crucial role of SLAMF9 in pDC differentiation， homeostasis， and function in the steady state and during experimental autoimmune encephalomyelitis.