Three oligomeric hydrolysable tannins, coriariins K, L, and M, which were previously undescribed, together with five known hydrolysable tannins were isolated from dried leaves of Coriaria japonica. Their structures were determined based on 1D and 2D NMR spectroscopy, HR-ESI-MS, and ECD spectroscopy experiments. Among the isolated compounds, coriariin M has a unique trimer structure where both dehydrodigalloyl and valoneoyl group linkages were found between the hydrolysable tannin monomers. Dimeric hydrolysable tannins coriariins K and L, having a dehydrodigalloyl group as the linking unit, were structurally related to coriariin A, the main hydrolysable tannin of this plant species. Additionally, the complexation of the eight hydrolysable tannins isolated in this study with bovine serum albumin (BSA) to form water-soluble macromolecules was analyzed using native polyacrylamide gel electrophoresis (PAGE). A comparison of the behaviors of the oligomeric hydrolysable tannins suggested the participation of the hexahydroxydiphenoyl group and the importance of the molecular sizes of the hydrolysable tannins in the formation of macromolecules.