Lithium metal dendrites have become a roadblock in the realization of next-generation solid-state batteries with lithium metal as high-capacity anode. The presence of surface and bulk inhomogeneities with non-negligible electronic conductivity in crystalline electrolytes such as the lithium garnet Li7La3Zr2O12 (LLZO) facilitates the growth of lithium filaments, posing a critical safety risk. Here we explore the amorphous phase of LLZO (aLLZO) as a lithium dendrite shield owing to its grain-boundary-free microstructure, stability against metallic lithium, and high electronic insulation. We demonstrate that by tuning the lithium stoichiometry in sputtered aLLZO films, the ionic conductivity can be increased up to 10-7 S cm-1 while retaining an ultralow electronic conductivity of 10-14 S cm-1. In Li/aLLZO/Li symmetric cells, plating-stripping results in no degradation of the films and current densities up to 3.2 mA cm-2 can be applied with no signs of lithium penetration. The defect-free and conformal nature of the films enables microbatteries with an electrolyte thickness as low as 70 nm, which withstand charge-discharge at 0.2 mA cm-2 for over 500 cycles. Finally, we demonstrate that the application of aLLZO as a coating on crystalline LLZO lowers the interface resistance and significantly impedes the formation of lithium dendrites, increasing the critical current density of a symmetric cell up to 1.3 mA cm-2 at room temperature and without external pressure. The effectiveness of the amorphous Li-La-Zr-O as lithium dendrite blocking layer can accelerate the development of more powerful and safer solid-state batteries.