Objective To investigate the role of human umbilical cord mesenchymal stem cells (hucMSCs) in the treatment of dextran sulfate sodium (DSS)-induced inflammatory bowel disease (IBD). Results ICG-hucMSCs homed to colon tissues of IBD mice 12 h after injection. The injection of hucMSCs significantly relieved the IBD symptoms and inflammatory cell infiltration. The expression of IL-10 gene increased while those of 15-LOX-1, TNF-alpha, IL-6, IL-1 beta, and IP-10 genes decreased in colon tissues and spleens of hucMSCs-treated mice. The activation of STAT3 was inhibited in colon tissues and spleens of IBD mice that were treated with hucMSCs. In addition, the percentage of macrophages decreased in colon tissues and spleens of hucMSCs-treated IBD mice. Moreover, we provided evidence that in vitro co-culture with hucMSCs inhibited the expression of 15-LOX-1, IL-6 and p-STAT3 in mouse enterocoelia macrophages. Conclusions HucMSCs alleviate DSS-induced IBD through the modulation of 15-LOX-1 in macrophages.