Ferroelectrics (FEs) are materials of paramount importance with a wide diversity of applications. Herein, we propose a postsynthetic methodology for the smart implementation of ferroelectricity in chiral metal organic frameworks (MOFs): following a single crystal to single-crystal cation metathesis, the Ca2+ counterions of a preformed chiral MOF of formula Ca-6(II){C-24(II)[(S,S)-hismox](12) (OHO2)}center dot 212H(2)O ( 1), where hismox is a chiral ligand derived from the natural amino acid L-histidine, are replaced by CH3NH3+. The resulting compound, (CH3NH3)(12){Cu-24(II)[(S,S)-hiSMOX](12)(OH2)(3)}center dot 178H(2)O (2), retains the polar space group of 1 and is ferroelectric below 260 K. These results open a new synthetic avenue to enlarge the limited number of FE MOFs.