From the fruit extract of Alnus sieboldiana, a new hydrolyzable tannin, alnusiin (1), was isolated together with five known tannins : tellimagrandin I (6), pedunculagin (7), casuarinin (9), casuariin (10) and 2, 3-O-(S)-hexahydroxydiphenoyl-D-glucose (11). The structure (1) of alnusiin in which a monolactonized tergalloyl group, a rare component unit, bonds to the glucose residue, was elucidated based on spectral analyses including ¹H-¹³C long-range shift correlation spectroscopy, and chemical methods. Another new hydrolyzable tannin with a related structure, bicornin (21), and six known galloylglucoses as well as six ellagitannins (including oligomers), were isolated from Trapa bicornis fruits and their structures were characterized.