In order to develop a potent antidementia $eta$-secretase inhibitor from phytochemicals, $eta$-secretase inhibitory activities of extracts from many medicinal plants and herbs were determined. Water extracts from Rubus coreanus showed the highest $eta$-secretase inhibitory activity of 84.5%. After purification of the $eta$-secretase inhibitor from R. coreanus using systematic solvent extraction, ultrafiltration, Sephadex G-10 column chromatography, and reverse-phase high performance liquid chromatography (HPLC), a purified $eta$-secretase inhibitor with $IC_{50}$ inhibitory activity of $6.3{ imes}10^3;ng/mL$ ($1.56{ imes}10^{-6};M)$ was obtained with a 0.08% solid yield. The molecular mass of the purified $eta$-secretase inhibitor was estimated to be 576 Da by liquid chromatography-mass spectrometry (LC-MS) and $eta$-secretase inhibitor also is a new tetrapeptide with the sequence Gly-Trp-Trp-Glu. The purified $eta$-secretase inhibitory peptide inhibited $eta$-secretase non-competitively and also show less inhibition on trypsin, however no inhibition on other proteases such as $alpha$-secretase, chymotrypsin, and elastase.