AbstractLactococcus lactis subsp. cremoris is a lactic acid bacterium commonly used in the cheese manufacturing industry. It is known to produce antibacterial peptides and has recently received attention for its role as a probiotic strain. Here, we report the isolation of a new strain, Lactococcus lactis subsp. cremoris RPG-HL-0136 (RPG0136) from dried compost, which exhibits strong antibacterial activity. When RPG0136 was fed to mice, it increased the intestinal population of two beneficial bacteria, Lactobacillus and Bifidobacterium, whereas it decreased the intestinal population of two harmful bacteria, Bacteroides and Enterobacter. In addition, it increased the concentration of short-chain fatty acids, including acetic acid, propionic acid, and butyric acid, with a simultaneous decrease in pH, and accelerated the catabolic degradation of proteins, lipids, and starch. Lastly, RPG0136 increased the plasma IgG and intestinal mucosal SIgA concentrations and upregulated Reg3r, MUC1, and MUC2 expression to improve the intestinal mucosal immune function. The results of this study suggest that RPG0136 is a potential probiotic strain that supports the growth of a beneficial microbiome by promoting the synthesis of organic acids and enhancing intestinal immune function.