Trigonella foenum-graecum (fenugreek) can ameliorate dyslipidemia, but the detailed mechanism is unclear. In this study, we examined the effects of fenugreek on hepatic lipid metabolism, particularly lipogenesis, which is enhanced in obesity and diabetes, in diabetic obese KK-Ay mice. KK-Ay mice were fed a control high-fat diet (HFD; 60% of energy as fat) (C group) or an HFD containing 0.5% or 2% fenugreek (0.5F and 2.0F groups, respectively) for 4 wk. Hepatic and plasma TG and mRNA expression levels of lipogenic genes were lower in the 2.0F group at 4 wk (P < 0.05), but not in the 0.5F group, than in the C group. The hydrolyzed saponin fraction, but not the saponin fraction per se, in fenugreek inhibited the accumulation of TG in HepG2 cells. We fractionated the hydrolyzed saponin into 15 fractions by HPLC and examined the effect of these fractions on TG accumulation in HepG2 cells. Fraction 11 inhibited TG accumulation in HepG2 cells and we determined by liquid chromatography tandem MS that the active substance contained in fraction 11 is diosgenin. Diosgenin (5 and 10 μmol/L) inhibited the accumulation of TG and the expression of lipogenic genes in HepG2 cells. Moreover, diosgenin inhibited the transactivation of liver-X-receptor-α, as measured using a luciferase assay system and by gel mobility shift assay. These findings suggest that fenugreek ameliorates dyslipidemia by decreasing the hepatic lipid content in diabetic mice and that its effect is mediated by diosgenin. Fenugreek, which contains diosgenin, may be useful for the management of diabetes-related hepatic dyslipidemias.