The garnet oxide Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte due to its high ionic conductivity, favorable elastic properties, and chemical stability against Li metal. Nevertheless, despite its high shear modulus, recent studies have shown that metallic Li ‘dendrites’ can penetrate LLZO through its grain boundary (GB) network, resulting in a short-circuit. This behavior suggests that microstructural features (such as GB) will play an important role in suppressing this failure mode. The present study examines the possibility for nanoscale softening (i.e., reduced elastic moduli) and reduced strength in the vicinity of GBs in LLZO. Molecular dynamics simulations were performed to predict the Young’s and shear modulus near two prototypical grain boundaries. The ideal shear strength and fracture toughness was also calculated for bulk and GB-containing systems to determine the impact of GBs on plastic deformation and the potential for Li penetration along GBs. Our calculations reveal that all three of these mechanical properties (elastic modulus, ideal strength, and fracture toughness) are reduced in the vicinity of GBs relative to the bulk. This behavior could potentially contribute to the observed penetration of Li along GBs in LLZO at high current densities. Similar behavior is anticipated at defects such as surface cracks or pores.