Despite the various biological activities exhibited by water chestnut (the fruit of the Trapa genus), the phenolic compounds present in its extract require comprehensive characterization. Accordingly, we analyzed a 80% methanol extract of commercially available water chestnut and identified a new hydrolyzable tannin dimer termed trapadin A. Additionally, 22 known compounds, including 10 hydrolyzable tannin monomers and 2 dimers, were also detected in the extract. Spectroscopic and chemical methods were used to elucidate the structure of trapadin A, revealing it to be a hydrolyzable tannin dimer formed from units of tellimagrandin II and 1,2,3,6-tetra-O-galloyl-β-d-glucose. Moreover, the 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity assay used to determine the half-maximal effective concentration values for the 23 compounds isolated from water chestnut indicated significant radical scavenging activity associated with hydrolyzable tannins. Notably, trapadin A, the new hydrolyzable tannin dimer, exhibited the highest activity value among the tested compounds.