Orange prismatic crystals of the first thallium hydrous nesosilicate Tl₄SiO₄·0.5H₂O have been obtained by evaporation from aqueous solution. There are three symmetrically independent Tl⁺ cations and five symmetrically independent oxygen atoms in the structure of Tl₄SiO₄·0.5H₂O. The O(4) and O(5) atoms belong to water molecules. Coordination polyhedra of the Tl ⁺ cations are strongly distorted because of the stereoactive behavior of lone electron pairs. The structure of Tl₄SiO₄.0. 5H₂O contains sheets of SiO₄ tetrahedra and Tl coordination polyhedra. The sheets have the composition [Tl₃SiO ₄]^- and are parallel to [100]. Within the sheets, SiO ₄ tetrahedra link to thallium polyhedra though common corners. The sheets are linked by dimers of face-sharing Tl(3)O₅ polyhedra, thus providing interconnection of the sheets into a framework. The framework has large elliptical channels occupied by water molecules (OW2) and electron pairs of Tl⁺ cations. The comparison with some other M⁺ (M = K, Ag, Tl) silicates is given.