Chromatin-modifying HDACi exhibit anti-inflammatory properties that reflect their ability to suppress DC function and enhance regulatory T cells. The influence of HDACi on MDSCs, an emerging regulatory leukocyte population that potently inhibits T cell proliferation, has not been examined. Exposure of GM-CSF-stimulated murine BM cells to HDACi led to a robust expansion of monocytic MDSC (CD11b⁺Ly6C⁺F4/80^intCD115⁺), which suppressed allogeneic T cell proliferation in a NOS- and HO-1-dependent manner with similar potency to control MDSCs. The increased yield of MDSCs correlated with blocked differentiation of BM cells and an overall increase in HSPCs (Lin^–Sca-1⁺c-Kit⁺). In vivo, TSA enhanced the mobilization of splenic HSPCs following GM-CSF administration and increased the number of CD11b⁺Gr1⁺ cells in BM and spleen. Increased numbers of Gr1⁺ cells, which suppressed T cell proliferation, were recovered from spleens of TSA-treated mice. Overall, HDACi enhance MDSC expansion in vitro and in vivo, suggesting that acetylation regulates myeloid cell differentiation. These findings establish a clinically applicable approach to augment this rare and potent suppressive immune cell population and support a novel mechanism underlying the anti-inflammatory action of HDACi.