The surfactant-like peptide (Ala)6-(Asp) (A6D) is shown to self-assemble into ultrathin (3 nm thick) nanosheets in aqueous solution above a critical aggregation concentration. A combination of circular dichroism and FTIR spectroscopy and X-ray diffraction shows that the nanosheets comprise interdigitated bilayers of the peptide with β-sheet conformation. The self-assembly can be modulated by addition of hexamethylenediamine which is expected to interact with the anionic C terminus (and C-terminal D residue) of the peptide. Multiple ordered nanostructures can be accessed depending on the amount of added diamine. Nanosheet and bicontinuous network structures were observed using cryogenic-TEM and small-angle X-ray scattering. Addition of hexamethylenediamine at a sufficiently large molar ratio leads to disruption of the ordered nanostructure and the formation of a solution of A6D-diamine molecular complexes with highly charged end groups. The multiple acid-functionalized nanostructures that are accessible in this system are expected to have many applications in the fabrication of new nanomaterials.