A new lead(ii) borosilicate, Pb6B2Si8O25 (1), has been synthesized by a high-temperature, high-pressure hydrothermal reaction at 480 °C and 990 bar. Its structure was determined by single-crystal X-ray diffraction. The reaction product was phase-pure as indicated by powder X-ray diffraction and whole pattern fitting using the Pawley method. Compound 1 has a 2D layer structure with the lead ions being located at interlayer regions. Each layer is formed of corner-sharing BO4 or SiO4 tetrahedra and contains an eight-ring window. The layer consists of a new fundamental building block (FBB) with the formula T8O23 (T: B or Si) formed by two (B(1)0.8Si(1)0.2)O4 tetrahedra and six (Si(2)0.933B(2)0.067)O4 tetrahedra. The FBB can be described as double open-branched triple tetrahedra. Another interesting structural feature of 1 is boron-silicon mixing which is uncommon in borosilicates. There are three unique tetrahedra in the structure: B(1)O4 tetrahedra with 20% substitution of Si for B, Si(2)O4 tetrahedra with 6.67% substitution of B for Si, and Si(3)O4 tetrahedra without substitution. We have applied 11B and 29Si MAS NMR spectroscopy to study the substitutional disorder. The NMR study results not only confirm the mixing of B and Si atoms in the structure, but also verify quantitatively the results from single-crystal X-ray diffraction. The reasons for boron-silicon mixing in the structure are discussed. Crystal data for 1: trigonal, R3c (no. 167), a = 9.5090(2) Å, c = 42.3552(8) Å, V = 3316.7(2) Å3, Z = 6, R1 = 0.0147, and wR2 = 0.0309.