Complex hydrides are a family of compounds which have attracted a lot of attention in the last decade for various clean energy-related purposes, from solid state hydrogen storage to materials suitable in Li-ion batteries. We present two new garnet-type borohydride materials suitable as solid state electrolytes. Li3K3Ce2(BH4)12 and Li3K3La2(BH4)12 show unexpectedly high room temperature Liþ ionic conductivity (compared to the reported isostructural garnet oxide Li-conductor) of sigma_Li 3 x 10**7 and 6 x 10**7 S/cm with corresponding activation energies of Ea = 0.79 and Ea = 0.67 eV, respectively, which result from large bottleneck windows in the conduction path. The effect of heterovalent cation substitution is investigated as means of tailoring ionic conductivity. Doping with divalent Sr2+ and Eu2+ shows that sigma_Li can be increased by one order of magnitude in the whole temperature range measured.