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In contrast to monovalent lithium or sodium ions, the reversible insertion of multivalent ions such as Mg and Al into electrode materials remains an elusive goal. Here, we demonstrate a new strategy to achieve reversible Mg and Al insertion in anatase TiO, achieved through aliovalent doping, to introduce a large number of titanium vacancies that act as intercalation sites. We present a broad range of experimental and theoretical characterizations that show a preferential insertion of multivalent ions into titanium vacancies, allowing a much greater capacity to be obtained compared to pure TiO. This result highlights the possibility to use the chemistry of defects to unlock the electrochemical activity of known materials, providing a new strategy for the chemical design of materials for practical multivalent batteries.