The rheological properties of the environment in which a cell lives play a key role in how the cells will respond to that environment and may modify cell proliferation, morphology and differentiation. Effective means of modifying these properties are needed, particularly for peptide hydrogels which are generally relatively weak and soft. In this report we describe the enzymatic cross-linking of a nanofibrous multidomain peptide hydrogel. When this method was used, the storage modulus, G', could be increased to over 4000 Pa without changes in hydrogel concentration and without dramatic changes in nanostructural architecture. Enzymatic cross-linking represents a mild and simple method for increasing the mechanical strength of peptide hydrogels in applications for which the robustness of the gel is essential. This method should be suitable for a broad array of peptide hydrogels containing lysine such as those currently under study by many different groups.