A new acentric protonated garnet Li6−xHxCaLa2Nb2O12 has been synthesized and structurally characterized from Rietveld refinement of high-resolution neutron diffraction data. This phase can be prepared by Li+/H+ exchange on the mother garnet Li6CaLa2Nb2O12 in acetic acid heated at reflux for 4 days, conditions determined after several tests varying acid solution, time, and temperature. Li6−xHxCaLa2Nb2O12 crystallizes in the noncentrosymmetric cubic space group I
3d (no. 220) with the cell parameter a = 12.8040(3) Å. The noncentrosymmetry has been confirmed from unambiguous results obtained by second harmonic generation test as well as from transmission electron microscopy study (selected area electron diffraction). The Li+/H+ exchange corresponds to a topotactic reaction since this new protonated phase is built from the garnet framework [CaLa2Nb2O12]6− like its mother form Li6CaLa2Nb2O12 with lithium and proton cations distributed on different sites.