<p>Glucocorticoids (GCs) are known for their anti-inflammatory potential, which includes the macrophage polarization into an anti-inflammatory and tissue remodeling state. GCs are routinely co-administered to cancer patients to alleviate the side effects of chemotherapy. However, it is not well known if GCs can modulate tumor-associated macrophages (TAMs) to promote tumor progression. Here, we show that dexamethasone (DEX) induces dose-dependent differentiation of THP-1 monocyte-derived anti-tumorigenic (M1) macrophages into pro-tumorigenic (M2-like) macrophages, even in the presence of M1 cues, and that DEX can repolarize fully differentiated M1 macrophages into an M2-like state. These macrophages have a cytokine profile similar to the pro-tumorigenic (M2) macrophages and can stimulate the proliferation and invasion of triple-negative breast cancer (TNBC) cells in vitro. DEX treatment of an orthotopic mouse model of TNBC attenuated paclitaxel-mediated tumor growth inhibition, increased M2-like TAMs in primary tumors, and enhanced lung metastasis. Transcriptomic analysis of DEX-treated M1 macrophages revealed not only transcriptomic overlap with M2 macrophages, but also with human breast cancer TAM transcriptomic data, and further to a specific TAM signature associated with aggressive estrogen receptor-negative breast cancer. Our study illustrates a remarkable macrophage repolarization plasticity upon DEX exposure that can promote tumorigenesis, warranting care in prescribing high doses of GCs to breast cancer patients, especially to those considered for chemotherapy.</p>