Mutually complementary, self-repulsive oligopeptide pairs were designed to coassemble into viscoelastic hydrogels. Peptide engineering was combined with biophysical techniques to investigate the effects of temperature on the structural and mechanical properties of the resulting hydrogels. Biophysical characterizations, including dynamic rheometry, small-angle X-ray scattering (SAXS), and fluorescence spectroscopy, were used to investigate hydrogelation at the bulk, fiber, and molecular levels, respectively. It has been found that temperature has a significant effect on the structure and mechanical properties of peptide-based biomaterials. Oligopeptide fibers assembled at 25 °C are formed faster and are two times thicker, and the resulting material is mechanically seven times stronger than that assembled at 5 °C.