A novel keratin-like fibrous protein K58 with molecular weight of about 58 kDa was discovered in bivalve Siliqua radiata ligament and identified by amino acid composition and MALDI-TOF-TOF analysis. We found that the protein is composed of cylindrical fibers (~160 nm in diameter) and contains high glycine (27.4%) and phenylalanine (10.5%) contents. Furthermore, it is homologous to keratin type II cytoskeletal 1, with repeat motifs of SGGG and SYGSGG. FTIR and secondary structure analysis indicate that K58 is composed of 46.2% β-sheet, 33.4% β-turn, 13.1% α-helix, and 4.7% disordered structure. This structure feature is closely related to the superior tensile strength, elasticity, and solvent resistance property of K58. These discoveries provide some evidence for evolution of keratin and fibrous proteins and prompt further studies of ligament fibrous proteins.