Crystals of a potassium-rich variant of the mineral tinsleyite were obtained by hydrothermal synthesis in the system K2O–Al2O3–P2O5–H2O. The new phase is monoclinic, space group P21/n, a 9.4352(7), b 9.5254(10), c 9.4309(7) Å, β 101.421(7)°, Z = 4. Its crystal structure was solved from single-crystal data collected at 193 K and refined to R = 0.0293, including positions of all hydrogen atoms. The crystal structure is built of units of four AlO6 octahedra sharing edges and vertices. The octahedron units are cross-linked by PO4 tetrahedra to form a three-dimensional framework with channels along the main crystallographic ,  and  directions, where the large K+ ions and H2O molecules are distributed. The system of O – H ... O asymmetric hydrogen bonds between H2O molecules, hydroxyl groups and framework oxygen atoms provides additional cross-linking in the structure. We show that a change in the alkalinity of the mineral-forming system correlates with the composition of the crystallizing phase and simultaneously influences the character of hydrogen bonding. The new K-rich variety of the tinsleyite is suggested as a possible mineral species. We evaluate the crystal chemistry of the leucophosphite group of minerals and the related synthetic compounds.