The new synthetic phase Mg₂Al₃O[BO₄]₂(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO₄], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40 kbar at temperatures that increase with pressure (∼650 → 900 °C), and it breaks down at higher temperatures to sinhalite + corundum + H₂O. At P ≥ 20 kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a = 7.455 (1) Å, b = 4.330 (1) Å, c = 9.825 (2) Å, β = 110.68 (1)°, and space group P2₁/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800 cm⁻¹ and 3500 cm⁻¹ in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10 kbar, 745 °C and 35 kbar, 950 °C, giving a slope of about 8 °C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO₂(OH).