CD84 is 1 of the 9 SLAM family cell-surface receptors involved in leukocyte activation. The CD84 ectodomain is highly glycosylated, and its cytoplasmic tail contains 2 copies of an ITSM, which can be phosphorylated. Here, we report that although mouse CD84 was present on all BM HSCs, its expression declined in developing thymic and BM lymphocytes. However, CD84 expression levels did increase significantly during the later maturation stages and were expressed abundantly on mature B and T cells. Among lymphocyte subsets, the highest expression was found on innate-like lymphocytes; specifically, on NKT and marginal zone B cells. Splenic CD4+ T(FH) cells exhibited higher levels of CD84 compared with the other CD4+ T cell subsets. CD84 was expressed abundantly on monocytes, macrophages, granulocytes, and DCs. Moreover, as the function of CD84 in myeloid cells remains unknown, we focused on the role this receptor plays in mouse macrophage activation. Transfection of CD84 in RAW-264.7 macrophages led to an increase in MAPK phosphorylation and NF-κB activation upon LPS stimulation. Concomitantly, the presence of CD84 increased the LPS-induced secretion of TNF-α and MCP-1 but lowered IL-10 and IL-6 production significantly. This modulatory effect was mediated by Y(300) within the second ITSM of CD84. Additionally, CD84 knock-down decreased TNF-α and IL-6 production in LPS-activated BMDMs. Taken together, these results show that mouse CD84 is a pan-leukocyte receptor, able to modulate signaling pathways downstream of TLR4, and regulates macrophage cell-fate decisions and effector functions.