<p>Solid electrolytes are fundamental to fuel cells, batteries, sensors and electrolysers. Among them, melilite oxides are promising oxide-ion solid electrolytes due to their unique layered tetrahedral networks. However, the defect chemistry in acceptor-doped melilites remains controversial and largely unchallenged for decades, particularly the long-standing assumption that oxygen vacancies can be created to prompt the oxide-ion transport. Herein, we provide robust experimental and theoretical evidence demonstrating that ionic transport in the acceptor-doped melilites is universally governed by interstitial cation migrations, rather than oxygen vacancies. In La_1-<i>x</i>Sr_{1+1.5<i>x</i>}Ga₃O₇, directional STEM-HAADF imaging, neutron and synchrotron X-ray powder diffraction, combined with pair distribution function analysis and reverse Monte Carlo modeling, demonstrate that the disordered interstitial Sr atoms in the average structure, together with correlated La/Sr disorder of local segregation in the local structure, enhance structural flexibility and create favorable cation migration pathways. High-fidelity machine-learning-potential molecular dynamics simulations further revealed that long-range Sr²⁺ migration is facilitated through a continuous “S-curve knock-on” mechanism between interstitial and lattice Sr sites. This study offers complementary insights into the defect chemistry and migration dynamics of interstitial cations in melilite solid electrolytes, laying a fundamentally important foundation of defect chemistry characterization for understanding ionic conduction and designing advanced solid electrolytes.</p>