LiBH4-based complex hydrides have attracted much attention as solid electrolytes that are highly compatible with Li metal anode, because of their reducing character and deformable nature. However, the compatibility of LiBH4-based complex hydrides following the formation of a stable interface with Li metal has not yet been fully verified. In this study, electrochemical and mechanical stabilities of LiBH4 and Li4(BH4)3I complex hydride solid electrolytes against Li metal were investigated. Li plating/stripping test performed for 100 cycles and cyclic voltammetry indicated their favorable electrochemical stabilities. Scanning electron microscopy and fast charge–discharge tests revealed the formation of a mechanically tight and durable interface between these solid electrolytes and the Li metal anode. To demonstrate the adequacy of the LiBH4-based complex hydride solid electrolyte for all-solid-state batteries with Li metal anode, an all-solid-state Li–S battery with a Li4(BH4)3I solid electrolyte was assembled and electrochemically tested. The battery showed reversible discharge and charge abilities. This study provides useful insights into a strategy that can be applied to develop complex hydride solid electrolytes.