Indoxyl sulfate is a uremic toxin and is difficult to remove by hemodialysis owing to its tight binding to albumin in the blood. There is therefore concern that it could cause inflammatory responses in renal tissue, leading to worsening of renal failure. Recent reports suggest that indoxyl sulfate not only directly affects renal cells but also induces an inflammatory response via macrophages infiltrating renal tissues. However, the mechanism of the indoxyl-sulfate-induced inflammatory response mediated by macrophages and the effect of macrophages on renal cells have not yet been elucidated. Here, we evaluated the effect of indoxyl sulfate on the inflammatory response of macrophages and the effect of macrophages on renal cells. Indoxyl sulfate upregulated the expression of tumor necrosis factor-α (TNF-α) by THP-1 macrophages. Moreover, it activated the transcriptional regulator nuclear factor-kappa B (NF-κB) p65, and an inhibitor of NF-κB suppressed indoxyl-sulfate-induced TNF-α elevation. Supernatant of THP-1 macrophages treated with indoxyl sulfate increased the mRNA expression levels of inflammatory cytokines in HK-2 renal proximal tubular epithelial cells. These results suggest that indoxyl sulfate increases TNF-α expression through the activation of NF-κB in THP-1 macrophages and that macrophages stimulated with indoxyl sulfate induce an inflammatory response in the proximal tubular epithelial cells.