The new mercury vanadium phosphate hydrate Hg4-xO1-y(VO)(PO4)(2).H2O has been synthesized under hydrothermal conditions. X-ray investigations led to orthorhombic symmetry, space group P2(1)2(1)2(1) (No. 19), a = 6.3632(2) Angstrom, b = 12.4155(5) Angstrom, c = 14.2292(6) Angstrom, Z = 4. The crystal structure was solved and refined from single-crystal diffractometer data to residuals R[F-2 > 2sigmaF(2)] = 0.039, R-W(F-2) = 0.055. The VPO framework consists of infinite one-dimensional [VO(PO4)(2)](infinity) chains with corner-connected VO6 octahedra and PO4 tetrahedra. The chains run along the [100] direction and are held together by the unprecedented tetrahedral cationic units [Hg4-xO1-y](4+), Presence of Hg-Hg bonding contacts is proved from theoretical calculations.