Monocytes in the tumor microenvironment (TME) can differentiate into immunosuppressive myeloid populations, including M-MDSCs and M2-like macrophages, thereby dampening T- and NK-cell–mediated anti-tumor immunity and limiting responses to cancer therapies. CD244 (2B4) is best known as an inhibitory receptor associated with lymphocyte exhaustion; however, its role in tumor-infiltrating monocyte/macrophage lineages remains incompletely defined. We hypothesized that CD244 functions as a macrophage-intrinsic immune checkpoint that restrains anti-tumor myeloid functions. Using a monocyte/macrophage-specific CD244 conditional knockout mouse (LysM-Cre; CD244^fl/fl^), we previously showed delayed tumor growth and increased anti-tumor myeloid differentiation, supported by mechanistic evidence that CD244 suppresses macrophage maturation and phagocytic/antigen-presentation programs (Jeongsoo Kim et al., Molecular Cancer, 2024). Building on these findings, we generated CRISPR–Cas9–engineered human CD244-knockout (KO) macrophage lines from THP-1 cells and evaluated anti-tumor functions in vitro and in vivo. CD244 KO THP-1–derived macrophages exhibited enhanced tumor-cell phagocytosis and increased antigen presentation–associated T-cell activation in vitro. In an A375 human melanoma xenograft model, intratumorally delivered CD244 KO THP-1 cells were detectable within tumors and displayed reduced PD-L1 expression compared with WT controls, consistent with a less immunosuppressive phenotype. Longitudinal tumor-volume monitoring indicated that CD244 KO THP-1–based treatment contributed to tumor growth control, with a trend toward improved efficacy under anti–PD-L1 combination settings. Collectively, these results support CD244 targeting as a strategy to functionally optimize macrophages—enhancing phagocytosis, antigen presentation, and TME remodeling—for developing myeloid-based immunotherapy against solid tumors.