A new acetophenone named rumexin $(3-hydroxy-5-methyl-4-O-{eta}-D-glucopyranosyl;acetophenone)$ was isolated from methanolic extract of Rumex aquatica together with eight known compounds, $quercetin-3-O-{eta}-D-glucuropyranoside$, $musizin-8-O-{eta}-D-glucopyranoside$, $quercetin-3-O-{alpha}-L-rhamnoside$, $emodin-8-O-{eta}- D-glucopyranoside$, caffeic acid, $1-O-caffeoyl-{eta}-D-glucopyranoside$, 1-methyl caffeic acid, $kaempferol-3-O-{eta}-D-glucuropyranoside$. All of the above compounds were isolated from Rumex aquatica for the first time, and structures of compounds were established by spectroscopic means.