A novel peptide, keramamide A 1, was isolated from the Okinawan marine sponge Theonella sp. collected off Kerama Islands. This study describes its isolation and structure elucidation. Keramamide A 1 is a unique hexapeptide comprising six amino acid residues: two leucine (Leu), two phenylalanine (Phe), one lysine (Lys), and a previously unknown amino acid, 6-chloro-5-hydroxy-N-methyltryptophan (MeCht). It also contains an unusual ureido bond formed by the α-NH groups of the Lys and one Phe residue. Isolation involved extraction with methanol-toluene (3:1), followed by flash chromatography on silica gel (methanol-chloroform 50:50), gel filtration on Sephadex LH-20 (methanol), and reversed-phase HPLC on ODS (methanol-water-trifluoroacetic acid 80:20:0.1), yielding 0.001% (wet weight). Structural assignment relied on extensive spectroscopic analyses including two-dimensional NMR (¹H-¹H COSY, NOESY, ROESY, COLOC, HMQC, HMBC, HOHAHA) and FAB MS/MS. HRFABMS determined the molecular formula as C₃₉H₆₁ClN₈O₉. NMR studies in [²H₆]-DMSO, chemical derivatization (diazomethane methylation to form methyl ester 2), and correlation experiments (NOESY, ROESY, COLOC, HMBC) established the amino acid sequence—including a cyclic pentapeptide moiety cyclo-(Phe¹-MeCht-Leu²-Leu¹-Lys) and the ureido linkage of Phe² to Lys—and supported the 2,5,6-trisubstituted indole structure of MeCht. Negative ninhydrin reaction indicated absence of an N-terminus, while positive Fast Red B salt reaction suggested a pyrrole chromophore.