The　framework　entanglements　show　structural　transitions　by　the　removal　and　incorporation　of　guest　molecules,　but　rarely　generate　phase　transitions　by　themselves.　In　this　study,　we　report　a　new　entangled　hydrogen-bonded　supramolecular　compound,　[(n-C4H9)(2)NH2]2H(2)C(4)O(4　center　dot)H(4)C(4)O(4)　(1,　H4C4O4　=　fumaric　acid),　which　undergoes　a　reversible　ferroelastic　phase　transition　with　the　Aizu　notation　of　2/mF　(1)　over　bar.　Differential　scanning　calorimetry　and　specific　heat　measurements　confirm　its　typical　second-order　phase　transition　at　around　228.8　K　(Tc),　while　the　results　of　the　deuterated　analogue　(2)　are　different　with　those　of　1,　indicating　that　proton　dynamic　motions　in　hydrogen　bonds　contribute　to　the　phase　transition.　Variable-temperature　single-crystal　X-ray　diffraction　analyses　reveal　that　the　cooperative　displacements　of　hydrogen　bonds　induce　the　structural　phase　transition,　which　arise　from　the　twisting　motions　of　the　fumaric　acid　molecules.　Simultaneously,　two　types　of　independent　hydrogen　bonding　layers　in　the　entanglement　are　altered　in　response　to　the　transformation　of　hydrogen　bonds　aggregates　at　the　low　temperature　phase,　causing　the　symmetry　breaking.　These　findings　will　open　up　a　new　avenue　for　the　design　of　ferroic　materials　with　an　entangled　framework.