New solid compounds A3−xGaO4H1−y (A = Sr, Ba; x ∼0.15, y ∼0.3), which are the first oxyhydrides containing gallium ions, have been synthesized by high-pressure synthesis. Powder X-ray and neutron diffraction experiments revealed that the series adopts an anti-perovskite structure consisting of hydride-anion-centered HA6 octahedra with tetrahedral GaO4 polyanions, wherein the A- and H-sites show partial defect. Formation energy calculations from the raw materials support that stoichiometric Ba3GaO4H is thermodynamically stable with a wide band gap. Annealing the A = Ba powder under flowing Ar and O2 gas suggests topochemical H− desorption and O2−/H− exchange reactions, respectively. © 2023 The Royal Society of Chemistry.