A new tellurium(vi) oxycompound, Ca10Na10[Te9O42](H2O), was synthesized by a higherature, high-pressure hydrothermal reaction at 550 °C, its structure determined by single-crystal X-ray diffraction, and further characterized by energy dispersive, infrared, and X-ray photoelectron spectroscopy. The compound crystallizes in the centrosymmetric trigonal space group R3c (no. 167) with a = 9.743(2), c = 66.337(13) Å, and Z = 6. The structure consists of discrete linear edge-sharing octahedral trimers with the composition Te3O1410-, which is a new type of finite Te-O oligomer with Te6+ only. The large complex anions form a hexagonal close-packed lattice with the octahedral and tetrahedral interstices being filled with Ca2+ and Na+ cations and water molecules. The octahedral trimers in every other h.c.p. layer rotate around the 31 screw axis for 120°; therefore the unit cell has a very long c axis. This work is one of the few examples of the synthesis of metal tellurates by supercritical hydrothermal techniques.