Abstract Tumor associated microglia and macrophages (TAMs) represent a main cell type of brain malignancies and demonstrate complex interactions with cancerous cells and the tumor microenvironment. These interactions have important implications for the progression and treatment of malignant central nervous system (CNS) tumors. In a non-oncological context, it has been shown that endogenous microglia can be replaced by peripheral hematopoietic cells by genetic or pharmacological intervention. A highly efficient replacement protocol (Shibuya et al, Sci Transl Med 2022, PMID:35294256) utilizing myeloablative bone marrow transplantation (BMT) followed by microglial depletion via pharmacological inhibition of the Colony-stimulating factor-1 receptor (CSF-1R) leads to almost complete repopulation of the CNS myeloid niche by circulation-derived myeloid cells (CDMCs). We investigated if this approach could be utilized to integrate CDMCs into brain tumors and study associated changes in the TAM signature. Microglia replacement in immune-competent mice was followed by intracranial transplantation of syngeneic cell lines of either high-grade glioma (GL261) or breast cancer metastasis (E0771). Lineage tracing proved high intra- and peri-tumoral chimerism of BMT-graft derived cells among CNS myeloid cells in both models two weeks after tumor cell injection. Tumor associated CDMCs (TA-CDMCs) showed a similarly activated morphology as naïve TAMs. We used single cell RNA sequencing of CD45-positive cells to further profile the transcriptomic identity of glioma TA-CDMCs. Pathway enrichment analysis revealed an upregulation of immune response pathways in TA-CDMCs compared to regular TAMs, including the overexpression of genes relevant for the regulation of T cell activation, suggesting possible modification of the glioma immune-environment. Strategies to manipulate the myeloid niche have the potential to provide further understanding of the pathobiology of TAMs and to build the basis for novel cell therapeutic approaches in the neuro-oncological domain.