This study describes the de novo design of pH-responsive protein filaments that assemble into micrometre-scale, well-ordered fibers at neutral pH. The filaments are composed of subunits containing six or nine buried histidine residues, which undergo protonation changes over a small pH range, leading to a sharp and reversible transition between assembled and disassembled states. Cryogenic electron microscopy (cryo-EM) reveals that the optimized design closely matches the computational model for both subunit geometry and fiber packing. Characterization using electron, fluorescent, and atomic force microscopy (AFM) shows a rapid disassembly process within 1 second after a pH drop. The pH transition midpoint can be tuned by modulating the number of buried histidines, demonstrating the potential for precise control over the material's response to environmental changes. This work highlights the feasibility of creating unbound nanomaterials that respond rapidly to small pH changes, opening new avenues for applications in tissue engineering, drug delivery, and self-healing biomaterials.This study describes the de novo design of pH-responsive protein filaments that assemble into micrometre-scale, well-ordered fibers at neutral pH. The filaments are composed of subunits containing six or nine buried histidine residues, which undergo protonation changes over a small pH range, leading to a sharp and reversible transition between assembled and disassembled states. Cryogenic electron microscopy (cryo-EM) reveals that the optimized design closely matches the computational model for both subunit geometry and fiber packing. Characterization using electron, fluorescent, and atomic force microscopy (AFM) shows a rapid disassembly process within 1 second after a pH drop. The pH transition midpoint can be tuned by modulating the number of buried histidines, demonstrating the potential for precise control over the material's response to environmental changes. This work highlights the feasibility of creating unbound nanomaterials that respond rapidly to small pH changes, opening new avenues for applications in tissue engineering, drug delivery, and self-healing biomaterials.