We report the design of an α-helical peptide that forms self-healing hydrogels at weight fractions below 0.1%. Gelation of the 21-residue peptide AFD19 is controlled by changes in pH, whereby highly-charged peptide states give low-viscosity solutions, while thermostable gels form at pH values where the peptide charge lies close to +1 or −1. At pH values where the molecular charge approaches zero, the peptide forms insoluble fibrils. Transitions between the liquid, gel and insoluble states of the peptide are reversible upon addition of acid or base. Electron microscopy, dynamic light scattering and electronic circular dichroism studies indicate that gelation occurs when the peptide self-assembles into soluble coiled-coil fibrils that form cross-links below a critical zeta potential. The formation of extended fibrils occurs in the absence of any explicit design to drive helix offsetting.