Tyrosine-containing peptide nanoassemblies have received tremendous attention because of their potential applications in biomedicine and nanomaterial fields. However, a current outstanding challenge is to direct the equilibrium between oxidative polymerization of precursors and the noncovalent assembly to precisely tune their specific nanostructures and functionalities through the rational design of peptide sequences. With a simple library of tripeptides containing tyrosine, glycine, and lysine, here we demonstrate how amino acid sequence encodes the property of tripeptide nanoassemblies by modulating the enzymatic oxidation of tyrosinase with the accompanied self-assembly, and thus select the pathways toward fluorescent or melanin-like nanoassemblies. The fluorescence of tripeptide nanoassemblies has been demonstrated in sensing both tyrosinase and melanoma. Our findings will provide inspiration of peptide sequence design for generating the complex bioactive peptide nanomaterials for biomedical applications.