Agastache rugosa, or Korean mint, is an herb used as a spice, food additive and traditional medicinal ingredient. It has desirable effects, such as its antibacterial, antifungal and antioxidant properties. A. rugosa contains many phenolic compounds studied for their various health benefits, with the primary components being tilianin. A. rugosa extract (ARE), which was extracted with ethanol and freeze-dried, contained 21.14 ± 0.15 mg/g of tilianin with a total polyphenol content of 38.11 ± 0.88 mg/g. Next, the antiadipogenic effect of A. rugosa and tilianin was clarified using 3T3-L1 cells, which differentiate into adipocytes and develop lipid droplets. 3T3-L1 cells were treated with ARE or tilianin and lipid accumulation (%) was calculated through oil red O staining. Tilianin elicited dose-dependent decrease in lipid accumulation (% of positive control) (30 μM 92.10 ± 1.19%; 50 μM 69.25 ± 1.78%; 70 μM 54.86 ± 1.76%; non-differentiation 18.10 ± 0.32%), assessed by oil-red-O staining, whereas ARE treatments caused consistent diminution in lipid accumulation regardless of dose (100 μM 86.90 ± 4.97%; 200 μM 87.25 ± 4.34%; 400 μM 88.54 ± 2.27%; non-differentiation 17.96 ± 1.30%), indicating that both compounds have anti-obesity effects on adipocytes. Treatment with ARE lowered the mRNA (PPARγ; C/EBPα; FABP4; SREBP1; ACC; FAS) and protein (PPARγ; C/EBPα; SREBP1) levels of adipogenesis and lipogenesis-related factors. Tilianin showed a greater effect on the mRNA levels compared with ARE. Thus, tilianin and ARE may have anti-adipogenic and anti-lipogenic effects on 3T3-L1 cells and be possible candidates of obesity-related supplements.