The crystal properties of a terbium silicate Na₂Tb_1.08Ca_2.92Si₆O₁₈H_0.8 synthesized at 1 GPa and 800 °C using a piston-cylinder apparatus is reported. Our single-crystal X-ray analysis shows that this new compound is triclinic with the space group 1 (Z = 1), and its unit-cell parameters are a = 7.056(3) Å, b = 7.075(3) Å, c = 8.029(3) Å, α = 102.381(5)°, β = 95.387(6)°, γ = 90.552(6)°, and V = 389.6(2) ų. This structure consists of octahedral double chains of the compositions [Ca_0.875Tb_0.125O₆] and [Ca_0.585Tb_0.415O₆]. The double chains are linked by tetrasilicate chains via vertex oxygen atoms to form a 3D framework with 6-membered (2 octahedra and 4 tetrahedra) ring channels along the b-axis where the Na⁺ cations locate. The luminescence properties of the Na₂Tb_1.08Ca_2.92Si₆O₁₈H_0.8 phase are also investigated. This phase, with most of its Tb in 3+, emits mainly from one transition, ⁵D₄ → ⁷F₅ (542 and 552 nm). The room temperature fluorescence decay curves are fit well by an exponential function, yielding a lifetime value of about 2.498(7) ms.