Predictable tuning of bulk mechanics from the molecular level remains elusive in many physical hydrogel systems because of the reliance on nonspecific and nonstoichiometric chain interactions for network formation. We describe a mixing-induced two-component hydrogel (MITCH) system, in which network assembly is driven by specific and stoichiometric peptide–peptide binding interactions. By integrating protein science methodologies with a simple polymer physics model, we manipulate the polypeptide binding interactions and demonstrate the direct ability to predict the resulting effects on network cross-linking density, sol–gel phase behavior, and gel mechanics.