The intestinal microbiota appears to play an important role in the development of atherosclerosis. We investigated the effect of the probiotic lactic acid bacterium Pediococcus acidilactici R037 on atherosclerosis using apolipoprotein E-deficient (ApoE ^−/−) mice. Six-week-old ApoE ^−/− mice were orally administered R037 six times a week. Mice treated with R037 for 12 weeks exhibited markedly attenuated atherosclerotic lesions in the aortic root (2.3 ± 0.15 × 10⁵ µm² vs. 3.3 ± 0.29 × 10⁵ µm², respectively; P < 0.01; n = 15–17 each group). The expression of Ki-67 in CD4⁺ T cells, the population of interferon γ-producing CD4⁺ T cells in the spleen, and pro-inflammatory cytokine production from splenic lymphocytes were significantly decreased in R037-treated mice. Interestingly, splenic dendritic cells (DCs) isolated from R037-treated mice suppressed CD4⁺ T-cell proliferation and pro-inflammatory cytokine production ex vivo, suggesting that R037 treatment induced tolerogenic DCs. Programmed cell death ligand 1 expression in DCs was significantly enhanced in R037-treated mice, which might explain the immunosuppressive effect of DCs at least in part. These results indicate that R037 attenuates atherosclerosis by inducing tolerogenic DCs, which suppress Th1-driven inflammation and the proliferative activity of CD4⁺ T cells. Our findings may provide a novel therapeutic approach for the prevention of atherosclerosis based on dietary supplementation with probiotics.