La2Mo2O9 is acknowledged as an exceptional oxide ion conductor. It undergoes a reversible phase transition around 580 degrees C from the nonconductive low-temperature monoclinic alpha-La2Mo2O9 phase to the highly conductive high-temperature cubic beta-La2Mo2O9 phase. In addition, La2Mo2O9 demonstrates complex chemistry under reducing conditions. This study reports, for the first time, the stabilization at ambient temperature of a novel cubic phase through a topotactic reduction of alpha-La2Mo2O9 employing CaH2. This phase contains approximately similar to 3 atom % oxygen vacancies relative to the nominal composition (La2Mo2O8.68(1)). The cubic symmetry is associated with a static distribution of these vacancies, in contrast to the dynamic distribution observed in the high-temperature cubic beta-La2Mo2O9 phase reported previously. Additionally, the material exhibits mixed-ion-electronic conduction, which expands its potential use in applications requiring both ionic and electronic transport.