Reaping the promise of human embryonic stem (hES) cells hinges on effective defined culture conditions. Efforts to identify chemically defined environments for hES cell propagation would benefit from understanding the relevant functional properties of the substratum. Biological materials are often employed as substrata, but their complexity obscures a molecular level analysis of their relevant attributes. Because the properties of hydrogels can be tuned and altered systematically, these materials can reveal the impact of substratum features on cell fate decisions. By tailoring the peptide displayed to cells and the substrate mechanical properties, a hydrogel was generated that binds hES cell surface glycosaminoglycans (GAGs) and functions robustly in a defined culture medium to support long-term hES cell self-renewal. A key attribute of the successful GAG-binding hydrogels is their stiffness. Only stiff substrates maintain hES cell proliferation and pluripotency. These findings indicate that cells can respond to mechanical information transmitted via GAG engagement. Additionally, we found that the stiff matrices afforded activation of the paralogous proteins YAP/TAZ, which are transcriptional coactivators implicated in mechanosensing and hES cell pluripotency. These results indicate that the substratum mechanics can be tuned to activate specific pathways linked to pluripotency. Because several different hES and induced pluripotent stem cell lines respond similarly, we conclude that stiff substrata are more effective for the long-term propagation of human pluripotent stem cells.