A new cyclic heptapeptide, hymenamide F (I), was isolated from an Okinawan marine sponge Hymeniacidon sp. and the structure was elucidated on the basis of spectroscopic data of [Mso]-hymenamide F (2). The absolute configuration of each amino acid residue was determined by Marfey's method. The conformation in the solution of 2 is discussed on the basis of NMR data as well as molecular calculation.