Ghrelin is an orexigenic peptide hormone produced in the stomach. The major active form is octanoylated ghrelin, which is modified with an n -octanoic acid at the serine-3 residue. Inhibition of octanoylated ghrelin production is useful for the prevention and improvement of obesity. We previously developed a cell-based assay system employing a ghrelin-expressing cell line, AGS-GHRL8, and found various compounds that decreased octanoylated ghrelin levels using this system. (−)-Epigallocatechin-3- O -gallate (EGCG) is a bioactive catechin in green tea and reportedly has an anti-obesity effect; however, it remains unclear whether EGCG inhibits octanoylated ghrelin production. Therefore, in this study, we investigated the effect of EGCG on octanoylated ghrelin levels in AGS-GHRL8 cells and C57BL/6J mice. EGCG significantly reduced the octanoylated ghrelin level in AGS-GHRL8 cells. In mice, three days of treatment with TEAVIGO^®, which contains 97.69% EGCG, lowered the plasma octanoylated ghrelin level by 40% from that in control mice. In addition, TEAVIGO^® reduced the mRNA expression of ghrelin and prohormone convertase 1/3, an enzyme responsible for the processing of proghrelin to mature ghrelin, in the mouse stomach, suggesting that the reduced expression of these genes may contribute to the inhibition of octanoylated ghrelin production. These results suggest a decrease in the octanoylated ghrelin level to be involved in the anti-obesity effect of EGCG, which thus has potential for the development of anti-obesity agents with ghrelin-lowering effect.