Synthesis of pure phase Mg_1.2Ti_1.8O₅ and MgTiO₃ nanocrystals has proven to be challenging. Here, pure phase Mg_1.2Ti_1.8O₅ and MgTiO₃ nanocrystals were prepared. Furthermore, a new magnesium titanate, Mg_1.2Ti_1.8O₅, was synthesized via a solution-based route for the first time. As hydrogen evolution photocatalysts, both pure phase Mg_1.2Ti_1.8O₅ and MgTiO₃ nanocrystals exhibit excellent hydrogen production efficiency. In comparison with pure MgTiO₃ nanocrystals, the asprepared Mg_1.2Ti_1.8O₅ nanocrystals exhibited four times as much photocatalytic hydrogen production activity, up to 40 μmol·h^–1. Photoelectrochemical analysis, including linear sweep voltammetry, transient photocurrent measurement, electrochemical impedance spectroscopy, and construction of Mott-Schottky plots, demonstrated that the enhanced photocatalytic activity was attributed to the large surface area, fast photoelectron transfer, higher carrier density, and efficient charge separation of the Mg_1.2Ti_1.8O₅ nanocrystals.