An unprecedented A₂M^IM^IIIX₆-type double perovskite adopting a fully hexagonal BaNiO₃-type structure, (piperidinium)₂[KBiCl₆], undergoes a 2/mF ferroelastic phase transition at 285 K with a spontaneous strain of 0.0615, arising from the order–disorder transition of organic cations together with the synchronous displacement of inorganic chains.