IL-15 is a cytokine critical for development, maintenance, and response of T cells, natural killer (NK) cells, NK T cells, and dendritic cells. However, the identity and distribution of IL-15-expressing cells in lymphoid organs are not well understood. To address these questions, we established and analyzed IL-15-CFP knock-in mice.We found that IL-15 was highly expressed in thymic medulla, and medullary thymic epithelial cellswith highMHCclass II expressionwere the major source of IL-15. In bone marrow, IL-15 was detected primarily in VCAM- 1+PDGFRβ+CD31-Sca-1- stromal cells, which corresponded to previously described CXCL12-abundant reticular cells. In lymph nodes, IL- 15-expressing cells were mainly distributed in the T-cell zone and medulla. IL-15 was expressed in some fibroblastic reticular cells and gp38-CD31- double-negative stromal cells in the T-cell zone. Blood endothelial cells, includingall high endothelial venules, also expressed high IL-15 levels in lymph nodes, whereas lymphatic endothelial cells (LECs) lacked IL-15 expression. In spleen, IL-15was expressed inVCAM- 1+ stromal cells, where its expression increased as mice aged. Finally, IL-15 expression in blood and LECs of peripheral lymphoid organs significantly increased in LPS-induced inflammation. Overall, we have identified and characterized several IL-15-expressing cells in primary and secondary lymphoid organs, providing a unique perspective of IL- 15 niche in immune microenvironment. This study also suggests that some stromal cells express IL-7 and IL-15 differentially and suggests a way to functionally classify different stromal cell subsets.