The spleen is a major site for the immunological responses to blood-borne antigens that is coordinated by cells of the mononuclear phagocyte system (MPS). The chicken spleen is populated with a number of different macrophages while the presence of conventional dendritic cells (cDC) has been described. However, a detailed characterization of the phenotype and function of different macrophage subsets and cDC in the chicken spleen is limited. Using theCSF1R-reporter transgenic chickens (CSF1R-tg), in which cells of the MPS express a transgene under the control elements of the chickenCSF1R, we carried out an in-depth characterization of these cells in the spleen. Immunohistological analysis demonstrated differential expression of MRC1L-B by periarteriolar lymphoid sheaths (PALS)-associatedCSF1R-tg+cells. In the chicken's equivalent of the mammalian marginal zone, the peri-ellipsoid white-pulp (PWP), we identified high expression of putative CD11c by ellipsoid-associated cells compared to ellipsoid-associated macrophages. In addition, we identified a novel ellipsoid macrophage subset that expressed MHCII, CD11c, MRC1L-B, and CSF1R but not theCSF1R-tg. In flow cytometric analysis, diverse expression of theCSF1R-tg and MHCII was observed leading to the categorization ofCSF1R-tg cells intoCSF1R-tgdimMHCIIinter−hi,CSF1R-tghiMHCIIhi, andCSF1R-tghiMHCIIintersubpopulations. Low levels of CD80, CD40, MHCI, CD44, and Ch74.2 were expressed by theCSF1R-tghiMHCIIintercells. Functionally,in vivofluorescent bead uptake was significantly higher in theCSF1R-tghiMHCIIhiMRC1L-B+cells compared to theCSF1R-tgdimandCSF1R-tghiMHCIIinterMRC1L-B+subpopulations while LPS enhanced phagocytosis by theCSF1R-tghiMHCIIintersubpopulation. The analysis of bead localization in the spleen suggests the presence of ellipsoid-associated macrophage subsets. In addition, we demonstrated the functionality ofex vivoderivedCSF1R-tg+MRC1L-BnegcDC. Finally, RNA-seq analysis of theCSF1R-tg subpopulations demonstrated that separating theCSF1R-tghisubpopulation into CD11chiand CD11cdimcells enriched for cDC and macrophage lineages, respectively, while theCSF1R-tghiMHCIIintersubpopulation was enriched for red pulp macrophages. However, our analysis could not define the cell lineage of the heterogeneousCSF1R-tgdimsubpopulation. This detailed overview of the MPS in the chicken spleen will contribute to future research on their role in antigen uptake and presentation.