A novel antifungal peptide, theonellamide F, was isolated from a marine sponge, genus Theonella. It is a dodecapeptide composed of L-Asn, L-aThr, two residues of L-Ser, L-Phe, βAla, (2S,3R)-3-hydroxyasparagine, (2S,4R)-2-amino-4-hydroxyadipic acid, τ-L-histidine-D-alanine, L-p-bromophenylalanine, and (3S,4S,5E,7E)-3-amino-4-hydroxy-6-methyl-8-(p-bromophenyl)-5,7-octadienoic acid. Its bicyclic structure including absolute stereochemistry was unequivocally determined as 1, which contains an unprecedented histidinoalanine bridge.