cis-Hinokiresinol, also known as (+)-nyasol, was isolated for the first time from an aquatic herbaceous plant, Trapa pseudoincisa NAKAI, via silica gel and octadecyl silica gel column chromatographies. The chemical structure was determined via analyses of the spectroscopic data, including NMR, MS and IR. cis-Hinokiresinol was also found to exhibit antioxidant and antiatherogenic activities. The $IC_{50}$ values for the scavenging activities of cis-hinokiresinol on ABTS cation and superoxide anion radicals were 45.6 and $40.5{mu}M$, respectively. The $IC_{50}$ values for the inhibitory effects on $Lp-PLA_2$, hACAT1, hACAT2 and LDL-oxidation were 284.7, 280.6, 398.9 and $5.6{mu}M$, respectively.