Polyethylene oxide (PEO)-based composite solid-state electrolytes with Li7La3Zr2O12 (LLZO) as a Li+ ion-conductive filler have recently attracted widespread attention due to their flexibility, easy processability, good contact with electrodes, and excellent electrochemical performance at high temperatures. However, the literature focuses on the cubic phase of LLZO as a filler due to its 2 orders of magnitude higher ionic conductivity than the tetragonal LLZO. Therefore, we synthesized a composite solid electrolyte based on PEO and tetragonal In-doped LLZO. The results showed that LLZO went through a phase transformation from tetragonal to cubic in PEO/LiTFSI at room temperature. Phase transformation is limited by the doping amount, suggesting that an excess amount of In in the LLZO structure inhibits the redistribution of Li+ ions needed for cubic phase stabilization. Moreover, this composite solid electrolyte showed a decrease in glass transition temperature and a wider electrochemical window of 5.3 V vs Li/Li+ compared to the PEO solid electrolyte. Composite solid electrolyte also exhibits a high Li+ ion transference number of 0.34 at 40 °C, a good cycling performance under the current density of 0.68 mA cm−2 at 40 °C, and an excellent Li+ ion conductivity of 2.40 × 10−5 S cm−1 at room temperature.