Tumor-associated macrophages (TAM) play a crucial role in promoting cancer progression. Upon cytokine stimulation, TAM preferentially polarize to the anti-inflammatory and pro-tumor M2 subtype. The mechanism underlying such preferential polarization remains elusive. Here, we report that macrophage-specific deletion of the SUMO-specific protease Sentrin/SUMO-specific protease 3 promotes macrophage polarization towards M2 in bone marrow-derived macrophage (BMDM) induced by interleukin 4 (IL-4)/IL-13 and in an ex vivo model (murine Py8119 cell line), as well as in a mouse orthotopic tumor model. Notably, Sentrin/SUMO-specific protease 3 (SENP3) loss in macrophages accelerated breast cancer malignancy in ex vivo and in vivo models. Mechanistically, we identified Akt Serine/threonine kinase 1 (Akt1) as the substrate of SENP3 and found that the enhanced Akt1 SUMOylation upon SENP3 loss resulted in Akt1 hyper-phosphorylation and activation, which facilitates M2 polarization. Analysis of clinical data showed that a lower level of SENP3 in TAM has a strong negative correlation with the level of the M2 marker CD206, as well as with a worse clinical outcome. Thus, increased Akt1 SUMOylation as a result of SENP3 deficiency modulates polarization of macrophages to the M2 subtype within a breast cancer microenvironment, which in turn promotes tumor progression.Keywords: Akt1; SENP3; SUMOylation; breast cancer; macrophage polarization.