Peptide amphiphiles represent a growing class of self-assemblers capable of forming a wide array of superstructures that can display targeted function. The electrostatic charge of the amphiphile is an important design attribute that can greatly influence the physical, mechanical, and biological properties of the resultant self-assembled material in which the peptide is contained in high copy number. We have previously reported the design of cationic amphiphiles that assemble into highly positively charged hydrogels. Herein, we report a new class of anionic beta-hairpin peptide amphiphiles that undergo pH and temperature-triggered folding and self-assembly into negatively charged fibrillar hydrogels capable of, shear-thin/recovery mechanical behavior. The influence of amino acid composition on material formation, local fibril morphology, and mechanical properties is defined and allowed the iterative design of gels capable of three-dimensional encapsulation of mesenchymal stem cells under physiological conditions.