This study investigated the effects of sake lees and yeast supplementation on rumen fermentation, nutrient digestibility, and blood metabolite levels in Japanese Black fattening cattle. Ten cattle were assigned to a 3×3 Latin square design with three treatments, including a sake lees group (300 g/head/day), yeast group (32 g/head/day of Levucell SC, 1.6 × 10¹⁰ cfu of yeast), and control group with no additive. Dry matter (DM) intake and DM digestibility were unaffected by treatment. In the sake lees group, molar proportion of acetic acid in rumen fluid decreased, whereas that of propionic acid increased. In the sake lees group, endotoxin (lipopolysaccharide [LPS]) activity in the rumen fluid was significantly higher than that in the yeast group. Rumen pH remained unaffected by the treatments. Yeast supplementation resulted in higher concentrations of non-esterified fatty acids and triglycerides in blood. The findings suggest that sake lees could shift rumen fermentation toward a more glucogenic profile, potentially improving energy utilization efficiency. Yeast supplementation did not show a clear effect on rumen fermentation; however, it induced changes in lipid metabolism, suggesting a potential contribution to the improvement of beef quality. Both supplements could be beneficial as functional feed ingredients in the diets of Japanese Black fattening cattle.