Breakdown of the intestinal epithelial layer's barrier function results in the inflow of commensal flora and improper immune responses against the commensal flora, leading to inflammatory bowel disease (IBD) development. Using a mouse dextran sodium sulfate (DSS)-induced colitis model, we show here that commensal Gram-positive bacteria trigger the mobilization of inflammatory monocytes and macrophages into the colon. Monocytes/macrophages are major producers of tumor necrosis factor-α (TNF-α), a representative cytokine that aggravates colitis. Notably, pretreating mice with vancomycin, which eliminated Gram-positive bacteria, particularly the Lachnospiraceae family, significantly reduced the severity of the colitis by selectively blocking the recruitment of monocytes/macrophages, but not of other cells. Importantly, vancomycin treatment specifically downregulated the colonic epithelial cell (cEC) expression of C-C chemokine receptor type-2 (CCR2) ligands, which are critical chemokines for monocyte/macrophage mobilization into the inflamed colon. Collectively, these results provide previously undiscovered evidence that Gram-positive commensal bacteria induce colitis by recruiting colitogenic monocytes and macrophages. Our findings may lead to new avenues of treatment for IBD.