Objectives: Even though widely known to be harmless, urea has been reported to impede the intracellular transport of arginine in vascular endothelial cells and attenuate glycolysis in pancreatic beta cells. In this study, we investigated whether high levels of extracellular urea affect macrophage functions. Methods: Peritoneal macrophages and bone marrow-derived macrophages (BMDMs) were treated with LPS and IL-4 for M1 and M2 polarization, respectively, in the presence of various concentrations of urea. Expression of arginase-1 and iNOS was examined by flow cytometry. Expression of M2 marker genes were also analyzed by qPCR. Seahorse real-time cell metabolic analysis was performed to check oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) of IL-4-treated BMDMs. Results: We found that physiologically high concentrations of urea attenuate the IL-4-induced upregulation of arginase-1 in murine macrophages in a dose-dependent manner at both mRNA and protein levels. Expression of other genes that are related to alternative activation of macrophages, such as Fizz1 and Ym1, was also significantly reduced by urea. On the other hand, urea had little effect on LPS-induced M1 polarization. Transcriptomic analysis of M2-polarized BMDMs by RNA sequencing revealed that urea tends to down-regulate the gene sets associated with mTOR signaling, glycolysis, and oxidative phosphorylation. In line with these results, phosphorylation of S6 protein and AKT, which are downstream targets of mTORC1 and mTORC2, respectively, was decreased by urea treatment. ECAR and OCR were also decreased in urea-treated M2 BMDMs. The urea-induced attenuation of M2 polarization was not obvious in the presence of Torin, a pan mTOR inhibitor, suggesting that the inhibitory effect of urea is mediated by downregulation of mTOR signaling. Conclusions: Based on these results, we propose that, contrary to the traditional knowledge assuming urea as an inert molecule, urea suppresses the M2 polarization of murine macrophages, likely via attenuation of mTOR signaling.