A new cyclohexasilicate, K2PbSi3O9.H2O, was synthesized under mild hydrothermal conditions. The crystal structure was solved by powder X-ray diffraction data. The unit cell is monoclinic, a = 6.5623(2), b = 11.7757(3), c = 13.0483(3) Angstrom, beta = 103.57(2)4 space group P2(1)/n, Z = 4, The structure consists of a framework of isolated Si6O18 rings with the Pb(IV) located in octahedral holes. Thus, the structural formula can be written as Pb2K4{uB, 6t}[Si6O18]. 2H(2)O. The structure encloses one eight membered monodimensional channel running parallel to the a axis of about 3.0 X 5.5 Angstrom (2). This channel is filled with one of the two crystallographically independent potassium cations and the water molecules. A second smaller six-membered ring channel along a is also present and contains the second K+. This compound represents a monoclinic version of the better-known orthorhombic members of the family.