This study investigated the effects of inorganic sulfur on metastasis in MDA-MB-231 human breast cancer cells. MDA-MB-231 cells were cultured in the absence or presence of various concentrations (12.5, 25, or 50 ${mu}mol$/L) of inorganic sulfur. Cell motility, invasion, and the activity and mRNA expression of matrix metalloproteases (MMPs) were examined. Numbers of viable MDA-MB-231 cells did not differ by inorganic sulfur treatment from 0 to 50 ${mu}mol$/L within 48 h. Inorganic sulfur significantly decreased cell motility and invasion in the MDA-MB-231 cells in a dose-dependent manner (P<0.05), as determined using a Boyden chamber assay and a Matrigel chamber. The activities of MMP-2 and MMP-9 were significantly reduced by inorganic sulfur in a dose-dependent manner (P<0.05). The inorganic sulfur also significantly inhibited MMP-2 and MMP-9 expression in the cells (P<0.05). These data suggest that inorganic sulfur can suppress cancer cell motility and invasion by inhibiting MMP-2 and MMP-9 activity and gene expression in MDA-MB-231 cells.