We report here the first unprotected D,L-tetrapeptide (Figure ) that assembles into a highly stable hydrogel at physiological pH and undergoes a thermally-induced irreversible re-organization from fibrils to plates. We support experimental data with all-atom MD simulations that focus not only on peptide-peptide but also on peptide-water interactions. Interestingly, the supramolecular re-organization is not dictated by a key conformational change of the peptide, but rather by an irreversible change in its hydration, revealing water as a key player in the process.