Abstract Intratumoral myeloid cells are highly heterogeneous in terms of development and function and are pivotal for forming and regulating the tumor microenvironment. However, the myeloid milieu in triple-negative breast cancer (TNBC) remains poorly understood. Here, to elucidate this myeloid milieu, we integrated in-house and public single-cell RNA sequencing data. We detected diverse neutrophil and mononuclear-phagocyte subtypes and delineated their developmental trajectories and functions.